I've reused OEM copies on multiple PC builds, but on only one at a time. I think an extremely strict interpretation of the EULA would say it can be used only once, but I've never had a problem with activating on a new build. If it won't activate over the internet and says you have to call, you'll usually get an automated response that just asks how many machines you have it installed on, and all you have to do is be honest and say 'one' and it'll give you a new activation code.
The worst part is having to read the long series of digits over the phone to the automated voice at the other end and then typing in the new code it reads back to you.
I've activated OEM copies of XP and Vista multiple times without problems.