In order to get Citrix clients prior to version 11 to function with Citrix XenApp running on Windows Server 2008 you will need to install a hotfix from Microsoft. 

Basically this has to do with the encryption that is used for terminal services licensing having changed its key size from 512 bytes to 2048 bytes.

Here is the link to the Microsoft Knowledge Base article: http://support.microsoft.com/kb/949914.  Unfortunately this is one of those hotfixes that you have to request from Microsoft.  The good news is that they aren’t making you jump through a lot of hoops to get it.  Mine came right away in an email!