If you own a Samsung Omnia 7 or a Samsung Focus, you may want to hold off performing the Windows Phone 7 update. A number of users have reported issues (going as far as bricked devices) after trying to do the update.
Microsoft is looking at a fix already and could have one out within the next few days. According to a post in their support forums:
We would recommend at this time, not to retry the update, as subsequent attempts will fail similarly. While we continue to investigate the issue, we would ask that you not attempt the update until your device alerts you of another update opportunity, in approximately 3 days’ time.
The issue appears to be related to Samsung firmware.