This happens to me all the time. It seems to happen if I accidentally linger too long while pressing certain keys, which brings up the alternate character options screen (o or n or e, etc.) which it's supposed to do, but sometimes instead of backing out of that and going back to typing, I close the alternate character box and my phone freezes. The physical keyboard doesn't work and although the buttons on the front seem like they're working (haptic feedback vibration) nothing happens and I have to reboot the phone. It's a pain and I haven't seen any fixes for it.
I'm also on Ally replacement number four and I absolutely hate this phone by now. I haven't had a single one that didn't have some kind of problem, so I gave up and stopped getting replacements and decided to live with this piece of crap. The Ally was a great phone before the Android 2.1 (Froyo) upgrade, which this phone is too clumsy and slow to handle. The processor sucks and can't keep up with anything. It makes sense since this phone was the cheapest Android phone on the market. Although I really like the Android OS, as soon as I have a discounted upgrade option, I'm switching to the iPhone 5. Or maybe a high-end Android phone.