For TFS 2005 users Deleting Builds are not a problem anymore.
For TFS 2008 users, Automate TFS Build Delete can be used as third party tool to easily manage your builds.
Simple statistics about all Builds and there status, fast way to enter Build Log, Build Drop Location, Changesets and of course delete them.
You can easily filter your Builds list by Date.( Example - Delete all builds before DD/MM/YYYY)