If you are part of the Vista Beta program then you might run into an issue of being prompted to activate after an upgrade. That in itself would not be a problem but there is a glitch with the new UAP (User Access Protection - I think) feature that causes activation to fail with an error 0x8007000D.
Fortunately you can work round this by disabling UAP.
I've seen write ups for this solution in several places but one of the clearest is how Richard Wu described the solution in a recent Technet Forum post:
1) Make sure the internet connection is active
2) Restart the PC. After the BIOS screen hit F8 a couple of times untill you see the Advanced Boot Options menu. Select Safe Mode and hit Enter.
3) Click on the Windows Key + R on the keyboard. This will open a RUN box. Type MSCONFIG and press Enter.
4) On MSCONFIG, select the TOOLS tab, scroll down and go to "Disable UAP". Hit the Launch button, close the command windows afterwards and click OK.
5) Restart the PC.
6) Logon to the PC with a user that has the Administrative permissions (If you have more than one account take the first account you created). Just like before when logging in as a user the "Activate Windows now" dialog appears. If not appear, use my above method to call the activation process out.
Note that the little shields are now gone from the options. Make sure you have internet connection and hit Activate now. Assuming your product key is correct you'll soon be smiling at the "Activation was successful" message.