My Kindle 3 has only rebooted on its own once while I've had it, so I wouldn't say spontaneous reboots were a normal factor of Kindle ownership, either. I was on 3.0.2 firmware at the time this reboot occurred, unlike many, I didn't have any problems with my Kindle when it had the 3.0.1 firmware, and it hasn't had any further crash and reboot problems, thus far, on 3.0.3.
If the problem continues with the latest firmware, Grimm, you certainly need to contact Amazon CS for a replacement.
And, as Tierstan stated, a weekly crash and reboot isn't normal, simaru; I don't think you should have to just live with it - perhaps you should contact Amazon CS again, too. Of course, that is totally up to you.
Holly