MS sometimes reset the online activation when you call them, other times they give you a code to type in... But your still using the same license key, and if you reinstall later 'online' activation should work again (In general).
Even XP/Vista retail editions will fail online activation if you change too much of your hardware spec, although Vista seems totally paranoid about minor changes MS really need to dial the protection down a notch, but according the their CEO they are planning to dial it up further.
Main difference with the Retail editions is 1) they reset the activation database every 120 days, so you get a free reinstall every 3 months, and secondly they are supposed to ask a lot less questions when you do a telephone reactivation of Retail.
Still is Retail worth 3x the price of OEM.. Thats up to you.