Hold the wake/sleep button until it gives you the message "Slide to Power Off". Slide it, then re-awaken the phone. When I did this, it stopped crashing.
Should I take it back for another one? I know this is minor, but small details like that bug me. It's $600 bucks so it's gotta be running well all the time.
Safari crashing is how my iPhone started on the fritz. Then other internet apps starting crashing, then settings starting crashing, then it started rebooting,....
Apple gave me a new iPhone today without much hassle really.