Twice. Once was Safari i think, it was pretty early on when I got it. The second one happened yesterday. The picture frame button on the home screen decided to crash the thing hard. Had a real hard time even getting it restarted.
Mine crashes consistently anytime I try to use Air Photo or File Browser to access large image files. One of them, I believe File Browser, kicks me back to the black screen with the silver apple that comes on when you first turn the iPad on.
No lock ups. My only problem with freezing is with Call of Duty, and its just the game that freezes. No problem to hit the home button to back out. I do have to restart the ipad if I want to play the game without error after that tho.
A few app crashes (Memeo Reader, Safari, Keynote and the Comcast iPhone app)
Repeatable spontaneous reboot which appears to be caused by opening a certain corrupted picture in the wallpaper picker (can do it over and over again with the same pic; none since deleting the image).
But no lock-ups.
So far more reliable than the XP boxes I have to use at work!
I don't have an iPad, but perhaps you could install the iPhone Configuration Utility on your computer, from there you can see the Console logs. Maybe something specific is happening each time that is causing the lockups.
Or you can take the easy road and just bring it in. I work in IT so I always try to find the root cause of problems.
I've had ZERO lockups on my launch day iPad. In fact, I haven't even powered the thing completely off yet, and I use it many hours per day. Of course, I've had quite a few apps crash to the home screen, but this is mainly due to developers not having actual hardware to test their apps on.