My team (back at MSFT) did this on checkin. It'd look at the class you changed and what tests had code coverage that touched that class, and rerun just those tests before allowing the commit. Post-commit, the rolling builds would update the code coverage for all of the different build flavors (debug, release, etc).

One nice side-effect of this support is that it also gave us code coverage deltas from the last build, so the system would send out embarrassing numbers with your commit if you'd dropped the overall coverage of the module.