Anybody running a cron job to rebuild MT on MediaTemple or any other hosting?

I want to set one up, but not sure how to.

The best option I know if is to use Tim Appnel's mt-rebuild script - you would just have your cron job call mt-rebuild. Unfortunately it is not certified to work on MT 4.x yet (as far as I know, anyway) so your mileage may vary.

Why?

Almost every time I've seen this question, the actual thing that needs to be solved is improperly-configured templates.

If you really do need to repeatedly schedule full rebuilds, though, Tim's script would be the way to do it. But I still can't think of many reasons it should ever be done.