Yes I have seen this happen. It vibrates and then you see the android boot screen. That and I've seen it hard crash, err well thats what I call it, since it just vibrates and I have to pull the battery out.
You can do a hard reset(Factory restore) and see if it happens again. Though if it starts to hard crash, I would call Tmo and start the process of a replacement. Note this isnt something they wil do right away, it has to be documented(ie you call in multiple times saying yes I've done the hard reset a couple of times and it comes back). In my case, I currently have a replacement in the mail.
But I would say to others, I think this is a rare thing and I still love this phone.