It's been confirmed with Microsoft directly that the RTM (not RC) build of Windows 7 will allow the user to run it unactivated for up to four months before requiring an activation key; a completely legal procedure.
"You can run the -rearm trick a total of three times," said Leonard. "If you perform a -rearm at the end of each 30-day period, you end up with 120 days of full, unfettered Windows 7 use, without having to supply an activation key."
Windows Secrets website explains that it's possible to install without needing to input an activation key (just hit next when prompted for one). You then have 30 days to activate, after which the OS will go into reduced functionality mode. But as explained above, it's possible to enter a basic command up to three times to renew the 30 day period.
By the time your four months are up, to save you the hassle of reinstalling the OS you can enter a legit Win7 key and continue on your merry way.