I've been having the same problem for over a year, but I've found out how to make it more tolerable. It's probably happening daily now. I replaced my first 8703 when it started happening, but have kept this one because I figured it's probably the same across the board. The first few times it happened to me, it would take literally 45 minutes of rebooting to become active again. However, the same was true if I took the battery out. I do about 5-6000 minutes/month sometimes and do a lot of email and text. I found that if I go in once per week and purge everything over a week old, it only takes 5 minutes for my phone to come back on.

I go to Messages and hit alt-p for phone, then I delete everything over 5 days.
I go to alt-O for outgoing and do the same
then I delete all sms's

I used the 8830 for a month and found that it was MUCH faster at deleting things.