Almost everyone is splitting their tests by browser; are we (IMVU) the only ones who have to horizontally shard because of test times?
Granted, our list of fully supported platforms is small (effectively IE6, IE7, IE8, and FF3.5) considering our downloadable client is currently windows-only.
If I'm understanding you correctly, TestSwarm handles this, actually. If you watch the video walkthrough you can see the tests being split apart and run against multiple browsers simultaneously. Not only does this allow the results to complete faster but it also helps to make error correction easier (only re-running a sub-section of tests to try and get a good response, rather than re-running the entire suite).
Granted, our list of fully supported platforms is small (effectively IE6, IE7, IE8, and FF3.5) considering our downloadable client is currently windows-only.