I received an email from an iPhone user greatly concerned about "either an unaddressed issue with both 2.0 and 2.01 software - or a rogue app(s) that is taking down the entire OS."
After visiting the App Store from his iPhone "nothing loaded, the screen just stayed white." After pressing the Home button and tapping the App Store again, nothing happened again. After rebooting the iPhone it hung on the black screen with the white Apple on it – permanently. What's scary is that a restore as new doesn't appear to fix the problem. He tried that four times.
BTL has the full text of the email and there's a growing thread about it on Apple's Support Discussion boards with more than 200 replies. If you've been bit by this bug you should also consider filing a bug report with Apple.
Update: One commenter on the PowerPage notes:
I've experienced this. What appears to happen is that the OS takes far longer to boot than normal. All I've had to do is have patience and wait for the login screen shortly after the two vibrations. This can take 3-5 minutes (I didn't time it, but time passed very slowly). My impatience sometimes lead me to believe that the iPhone was bricked and to force another restart. However, after the first, long, really long, incredibly long reboot, the iPhone seems to boot normally. Just be patient!
Update 2010-0416: If waiting doesn't work, the solution is to hold home and power for 20+ seconds until you see the "connect to iTunes" screen, pictured below: