If you are building 2008 solution that contains a web project that has a 32 bit dll reference and a WDP project , on 64 bit TFSbuild 2010, you might see that when the WDP project is built its calling the 64 bit version of aspnet_compiler, but the dependant web project is build as 32 bit so throws an error

Resolution:

On the process tad of build definition, you need to set "MSBuild Platform" to "x86" which will force it to pick up 32bit version of MSbuild. By default its set to "Auto"