Turn off label deletion when deleting builds
By default a build label is deleted when the corresponding build gets deleted.
In TFS 2008 SP1 you can change that behavior by modifying the web.config file (on the TFS AT in folder %ProgramFiles%\Microsoft Visual Studio 2008 Team Foundation Server\Web Services) to:
<add key="PreserveLabelsOnBuildDeletion" value="true" />
(Thanks to Buck Hodges for this tip).