In the two years that I've had my 9300b I had never seen this before though I had read about it on several forums: an unresponsive 9300 with nothing but a white screen.
Luckily, I was looking right at it when it occured so I am sure what caused it.
I was in a meeting and had my profile set to "Meeting", but timed to expire at 4:15 pm and revert back to "Normal".
I also have a daily AT&T Alert (subscription text message) set to come in at 4:15 pm with the "Joke of the Day".
It seems that the two NMI (Non-maskable Interrupts) caused by the internal timed profile change coupled with the incoming SMS message caused the Symbian 80 OS to hang--one task interrupted the other and it couldn't figure out where to return to.
A quick removal of the battery cover, removal of the battery for a few seconds, a slow reboot and I was back to business as usual.
Those of you who have reported white screen behavior before...what was your phone doing when it happened? Were you in a video or audio playback when an incoming message might have interrupted the CPU intensive task?
I've even read of the white screen problems on the E90, so it must be an OS interrupt priority issue. Nokia are you listening?