At last news, Beta 1 and Beta 2 of Office 2010 will time-bomb (expire) on October 31, 2010.

You can uninstall Office a few times before it will not let you activate it, but this is just a security feature. There is really no limit to the amount of times you can uninstall and reinstall. If activation does not work it will give you a way to activate by phone. Call the number and explain that you had uninstalled and reinstalled and you should be fine. As long as you are not trying to install the software on more computers than the license states.