My first foray into the world of blogs!
For some unknown reason visual studio has stopped showing me .Net 3.0 assemblies in the list generated when you right click -> add reference.
A quick google found me this:
http://support.microsoft.com/kb/306149
Following on from this, a quick squiz at another vista machines registry shows me this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\AssemblyFolders\Microsoft .NET Framework 3.0 Reference Assemblies for Visual Studio
which contains the following default value:
c:\\Program Files\\Reference Assemblies\\Microsoft\\Framework\\v3.0\\
Well, that path exists on my machine and the dlls are there too, woohoo ;-)
Adding the above key and default value, then closing and re-opening VS solved my problem.