Our Visual Studio Chief Architect, Rico Mariani, wrote up a great blog about why we haven't moved to a 64 bit version of Visual Studio. 

I've been asked this several times from partners and customers and I've gotten different responses asking the same question to other folks in the developer division.  Atleast Rico has put into perspective why we're not moving in that direction at this time.

This is definitely worth a few minutes to read:

http://blogs.msdn.com/ricom/archive/2009/06/10/visual-studio-why-is-there-no-64-bit-version.aspx