VCBUILD : error Message: '9.00' violates enumeration constraint of '7.00 7,00 7.10 7,10 8.00 8,00'

I upgraded a Visual C++ project from Visual Studio 2005 to 2008 today, and my MSBuild script stopped working. It spat out the error message below and Google was no help.

VCBUILD : error Message:
'9.00' violates enumeration constraint of '7.00 7,00 7.10 7,10 8.00 8,00'

I made two changes that seemed to fix the problem:

1. Use the MSBuild.exe in \Windows\Microsoft.NET\Framework\v3.5\ instead of \Windows\Microsoft.NET\Framework\v3.xx.

2. Pass MSBuild the "/toolsversion:3.5" flag.

I hope someone out there finds this useful.