I can say that I was getting this issue, it was frustrating, certainly at seeing the response. Having worked this through, the problem exists in the Microsoft.TeamFoundation.Build.Client file. I noticed a difference in the version on my build agent and also in the AssemblyReferences folder.
I upgraded my TFS Build Agent to SP1, and I am not getting the data from this method. My suggestion is to try this option, since there is VS2008 SP1 and there is TFS2008 SP1. The Build agent uses TFS2008 SP1.
TFS2008 SP1
http://www.microsoft.com/en-us/download/details.aspx?id=6097
Would be curious to know what the result is.