I've been experiencing the same thing and oddly enough, it has occurred at the same place, same time a few times now. On Fridays after work, I drive to my gym, playing songs on my iPhone in the car (via my Blitzsafe iPod adapter). I play volleyball at my gym and they have a free wireless network. At some point in the evening I will go to check my phone for messages or emails and when I unlock the phone, it gets stuck mid-unlock and I have to hold the buttons down a long time to get it to reset. And the reset doesn't always work the first time. Sometimes it will freeze again. Last night when this happened, it felt like since the autosleep is set to a minute, the moment I would get the phone to reset, it would be time for it to autosleep, so I wonder if that is a contributing factor. Is the phone trying to go to sleep at the same time I am unlocking it? I'm usually not that slow at unlocking it...
I'm fairly sure the battery level is fine. I do notice that the wifi signal has trouble making it into the gymnasium where we actually play. If I step out into the hallway, the signal is stronger. The reason I mention that is because I have noticed that the iPhone has trouble with wireless networks sometimes, causing it to act strangely. So, could it be that I was playing songs earlier? Could it be that the iPhone doesn't really like the car adapter? Could it be that the iPhone was getting confused by the weak wireless signal?
When this happens, I will turn off/on my iPhone and I'll reset my iPhone a couple of times in an attempt to clear up whatever was troubling it. And then the phone works fine for awhile. I've never tried restoring it, since the problem appears to go away for awhile. But I do find it odd that this issue occurs at the same location. Could be coincindence though, since I'm there regularly.