It would take a very wealthy software house to upgrade their system, just so they can use Linq. Stored procedures are superior to Linq, and I find them easier to construct.
If you do need WCF, then you can use VWD Express, but likelyhood now, is you are already deeply involved in ASMX web services already, so changing your service layer again would require a wealthy software house, and customers that like their systems upgraded willy-nilly.
I have Pro
C# 2005 and 2008 by Andrew Troelsen, and doubt I'm in a hurry to get Pro C# 2010. .NET is moving far to fast at present, and I still find enormous value in .NET 2.0.
I too develop in VS 2008, but from a head-versus-heart, i.e. business sense, I find it impossible to justify moving to .NET 4.0, if you are targeting XP using Windows Forms.