In sharepoint designer’s workflow editor how to get the workflow initiators username

In Sharepoint designer's workflow editor I wish to retrieve the username/name of the work flow initiator (i.e. who kicked it off or triggered the workflow) – this is relatively easy to do using 3rd party products such as Nintex Workflow 2007 (where I would use something like {Common:Initiator}) – but I can't seem to find any way out of the box to do this using share point designer and MOSS 2007.

Update

It does not look like this rather obvious feature is supported OOTB, so I ended up writing a custom activity (as suggested by one of the answers). I have listed the activities code here for reference though I suspect there are probably a few instances of this floating around out there on blogs as it's a pretty trivial solution:

public partial class LookupInitiatorInfo : Activity
{
    public static DependencyProperty __ActivationPropertiesProperty =
        DependencyProperty.Register("__ActivationProperties",
        typeof(Microsoft.SharePoint.Workflow.SPWorkflowActivationProperties),
        typeof(LookupInitiatorInfo));

    public static DependencyProperty __ContextProperty =
        DependencyProperty.Register("__Context", typeof (WorkflowContext),
        typeof (LookupInitiatorInfo));

    public static DependencyProperty PropertyValueVariableProperty =
        DependencyProperty.Register("PropertyValueVariable", typeof (string),    
        typeof(LookupInitiatorInfo));

    public static DependencyProperty UserPropertyProperty = 
        DependencyProperty.Register("UserProperty", typeof (string),
        typeof (LookupInitiatorInfo));

    public LookupInitiatorInfo()
    {
        InitializeComponent();
    }

    [Description("ActivationProperties")]
    [ValidationOption(ValidationOption.Required)]
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public Microsoft.SharePoint.Workflow.SPWorkflowActivationProperties __ActivationProperties
    {
        get { return ((Microsoft.SharePoint.Workflow.SPWorkflowActivationProperties)(base.GetValue(__ActivationPropertiesProperty))); }
        set { base.SetValue(__ActivationPropertiesProperty, value); }
    }

    [Description("Context")]
    [ValidationOption(ValidationOption.Required)]
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public WorkflowContext __Context
    {
        get { return ((WorkflowContext)(base.GetValue(__ContextProperty))); }
        set { base.SetValue(__ContextProperty, value); }
    }

    [Description("UserProperty")]
    [ValidationOption(ValidationOption.Required)]
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public string UserProperty
    {
        get { return ((string) (base.GetValue(UserPropertyProperty))); }
        set { base.SetValue(UserPropertyProperty, value); }
    }

    [Description("PropertyValueVariable")]
    [ValidationOption(ValidationOption.Required)]
    [Browsable(true)]
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
    public string PropertyValueVariable
    {
        get { return ((string) (base.GetValue(PropertyValueVariableProperty))); }
        set { base.SetValue(PropertyValueVariableProperty, value); }
    }

    protected override ActivityExecutionStatus Execute(ActivityExecutionContext executionContext)
    {
        // value values for the UserProperty (in most cases you
        // would use LoginName or Name)

        //Sid
        //ID
        //LoginName
        //Name
        //IsDomainGroup
        //Email
        //RawSid
        //Notes

        try
        {
            string err = string.Empty;

            if (__ActivationProperties == null)
            {
                err = "__ActivationProperties was null";
            }
            else
            {
                SPUser user = __ActivationProperties.OriginatorUser;

                if (user != null && UserProperty != null)
                {
                    PropertyInfo property = typeof (SPUser).GetProperty(UserProperty);
                    if (property != null)
                    {
                        object value = property.GetValue(user, null);
                        PropertyValueVariable = (value != null) ? value.ToString() : "";
                    }
                    else
                    {
                        err = string.Format("no property found with the name \"{0}\"", UserProperty);
                    }
                }
                else
                {
                    err = "__ActivationProperties.OriginatorUser was null";
                }
            }
            if (!string.IsNullOrEmpty(err))
                Common.LogExceptionToWorkflowHistory(new ArgumentOutOfRangeException(err), executionContext,
                                                     WorkflowInstanceId);
        }
        catch (Exception e)
        {
            Common.LogExceptionToWorkflowHistory(e, executionContext, WorkflowInstanceId);
        }

        return ActivityExecutionStatus.Closed;
    }
}

And then wire it up with the following .action xml file:

<?xml version="1.0" encoding="utf-8"?>
<WorkflowInfo Language="en-us">
<Actions>
    <Action Name="Lookup initiator user property"
 ClassName="XXX.ActivityLibrary.LookupInitiatorInfo"
 Assembly="XXX.ActivityLibrary, Version=1.0.0.0, Culture=neutral, PublicKeyToken=XXX"
 AppliesTo="all"
 Category="WormaldWorkflow Custom Actions">
        <RuleDesigner Sentence="Lookup initating users property named %1 and store in %2">
            <FieldBind Field="UserProperty" DesignerType="TextArea" Id="1" Text="LoginName" />              
            <FieldBind Field="PropertyValueVariable" DesignerType="ParameterNames" Text="variable" Id="2"/>
        </RuleDesigner>
        <Parameters>
            <Parameter Name="__Context" Type="Microsoft.Sharepoint.WorkflowActions.WorkflowContext, Microsoft.SharePoint.WorkflowActions" Direction="In"/>
            <Parameter Name="__ActivationProperties" Type="Microsoft.SharePoint.Workflow.SPWorkflowActivationProperties, Microsoft.SharePoint" Direction="In"/>
            <Parameter Name="UserProperty" Type="System.String, mscorlib" Direction="In" />
            <Parameter Name="PropertyValueVariable" Type="System.String, mscorlib" Direction="Out" />
        </Parameters>
    </Action>
</Actions>
</WorkflowInfo>