Linux – VPN compatible with iPhone/iPad WITHOUT GRE protocol

I'm not sure if any of these meet your needs, but TechNet has a blog where they tested several clients for compatibility.

VPN Client Compatibility with Windows 7 and Windows Server 2008 R2

According to them a version for Windows 7 of SafeNet SoftRemote is due out in Q4 2009.

In addition to TCP port 1723, you're going to need to enable IP protocol 47 to pass through your router/firewall -- this is for GRE, and could be the source of the error.

Can you describe your network further? You have a windows 2003 box w/ one NIC, so it sounds like it's connected directly to your router which is performing nat and has firewalling capability (which is what you've enabled port 1723 to be forwarded through)? Is this about right?

Additionally, is there any security software (i.e., a suite with a personal firewall) installed on the box that may be interfering with the windows firewall?