Mine has rebooted on it's own twice - I've had it for about two and half weeks. If it keeps doing it, I'll have it replaced.
My iphone turned itself off (not a reboot) twice over 6 months, and the second time I couldn't get it to turn back on. I took it into my local store and they were able turn it back on, but they replaced it anyway, right there in the store. I'm hoping that once Apple is caught up on stock that we'll be able to get in-store watch replacements, because if my watch keeps rebooting, I'm going to want a replacement, and I'd rather not do the mail-in thing.