For Team Build users, I'll spare you the suspense and tell you that there are no meaningful changes in Team Build itself in SP1. There are changes in other parts of the system, though, that will be of some benefit.
Version control users will get a number of fixes, but most are related to scalability.
Brian Harry described many of the TFS SP1 changes in a post in June. I'm sure he'll post a follow up soon now that SP1 beta has shipped.
You can find out how to get the SP1 beta at Microsoft Connect. Soma's post provides a higher-level description of what's in SP1 beta.
UPDATE: Rob Caron has posted the details of what's in SP1 beta.
tags: tfs, team foundation server, visual studio, vsts