With Windows Server 2008 Terminal Services you can utilise the Terminal Services Gateway Server to allow RDP access over 443, eliminating the need for VPNs or firewall re-configurations for external users.
On windows, you need the RDP client 6.1 to use TS Gateway Server.
On Macs, it appears you are out of luck: Microsoft's RDP client for Mac v2 does not support Gateway Server.
Is there any RDP client available for Macs that support Gateway Server?
Mac – TS Gateway support for Mac
macrdpterminal-serverwindows-terminal-services
Best Answer
I don't necessarily have a definitive answer but I can tell you that I've had cases open with Microsoft Support to deal with various implementation problems where the initial MS response was to use TS-Gateway and RemoteApp. Which I had to bring up that my non-windows clients would be excluded from this solution. In none of these cases were we able to find a Mac or Linux client that would provide access to these features. Further my requests for a road map on the Mac client support have as of yet gone unanswered.
There was some hope I found for a possible work around with an unsupported beta Silverlight version of the TSWeb services. Unfortunately, I never was able to work through testing or implementation of this. Various other posts and docs indicate it should be possible to allow non-windows clients to be able to connect to a farm essentially through a ts-gateway and even leveraging Remote-App. But I can't confirm or deny this to be true.
http://wss-id.org/blogs/narenda/archive/2008/01/03/ts-web-access-with-silverlight-beta.aspx
I keep hoping I've missed something but so far this is all I've found on the subject.