Welcome to the Android world. I've yet to find an Android smartphone (or rather OS version) on which you never (or almost never) experience some auto-reboot at some point.
I've experienced auto-reboot on multiple Android smartphones and version, including HTC Magic, HTC Desire, Samsung Galaxy S II and now Sony Xperia S, running various version of Android, from 1.6 to 4.0.4 passing through 2.1, 2.2 and 2.3. On all of them I've experienced automatic reboot.
Hopefully it's not too often and it's bearable, but it's not good for the image of Android, and Google should really work on improving significantly the stability of the system, as it still looks like a beta or RC version rather than really final one.
It's not that much surprising, it seems to be a common practice today to deliver first and fix later, and so use the public to betatest software. I don't like this philosophy very much, but sadly, until it changes one day, I've to cope with it as almost every company in the industry is adopting this kind of strategy (probably because it's more profitable and the companies reduce significant costs using this strategy without losing much on the customers side as customers are quite tolerant being used to that for years now).
