Absolutely no problems.
But consider this -
Unlike retail, OEM's don't come with colourful boxes and detailed manuals and CD's. And like aadipa said, the license is non transferable. It's tied to the hardware it came with. I think Windows activation deletes all records after 120 days. So if you do a major hardware upgrade, you should be fine reactivating as long as the previous activation was done before 120 days.
More info here - *www.microsoft.com/licensing/default.mspx
Keith