I tried to build a WPF XBAP that called a WCF service from its partially trusted environment using VS 2008 Beta 2 and managed to make it work but there's a bit in the docs that you need to read if you're playing with this stuff.

I don't have an online reference for it but the local MSDN topic is;

ms-help://MS.MSDNQTR.v90.en/wcf_con/html/a36a540b-1606-4e63-88e0-b7c59e0e6ab7.htm

if that's of any use. The topic is called "Feature Compatibility when Running Partially Trusted".

Essentially, the topic says;

1. Use BasicHttpBinding, WebHttpBinding, WSHttpBinding (transport security only)
2. Don't use MTOM
3. Don't use reliable messaging, transactions, secure conversation.
4. Some other limitations around serialization.
5. And some other bits and pieces.

What I also found was that if you try and configure via a Config file then you might (i.e. I did) hit an issue when you hit your <bindings> section as that seemed to bring into play an assembly that didn't allow partially trusted callers for me so I ended up avoiding any <bindings> section in my config file.