ML - Magic Lantern has been an ongoing software mod for the Canon 5D Mark II & now Canon T2i / 550D. Here's a video from Vimeo member Moira O'Brien showing off some of the latest mods some DSLRs lacked in it's release. Lots of cool tools thrown in that not even Canon's latest T3i has, but keep in mind that this modifies the camera's original design (not permanently).
When running the ML software on your camera, you're asking it to do things it wasn't originally designed to do. Is it safe? I've never heard any complaints, i've never head of any cameras failing because of the Software, but the T2i was notorious for over heating issues on short recordings. One thing I don't think i've ever recalled seeing is a temp gauge as part of Magic Lantern's features. As always, you do this on your own risk.
I personally am no longer using Magic Lantern software just because it didn't fit into the work flow. When it was first released, the software needed to remain on the media card (or all the cards you are using) in order to boot up from it. This means you can't quickly format your cards. I do totally support the Magic Lantern idea, I think it's awesome and it's developers like these that give more ideas to the Manufacturers about how they need to step up their game, and add these new features to future models. These types of mods changed the way people saw the Panasonic GH1. In fact, ML was out before Canon released a firmware update for the 5D Mark II to add manual audio levels. We'll just have to wait and see how many more of these other ML features are implemented into newer cameras.