I spent the night unpacking WhatsApp 2.0.0 XAP file fro Windows Phone 7 in search for useful information about certificates and encryption.
Anyways, didn't come to far as half the stuff is compile in DLLs and .NET Reflector didn't want to install for me.
If someone is using Reflector and can decompile these DLLs, drop me a PM, I'll provide the files extracted.