Yeah, I've reformatted my main machine a couple times now, with no problems getting it to activate.
Even if you do have problems doing the automatic activation online, you can do it over the phone. Had to do it the other day, when (due to my own stupidity) I re-installed XP on a new file server computer I built, for the 3rd time in a week (don't ask!!). Tried doing the ol' voice activated phone activation, and couldn't get that to play nice with me.
Next thing ya know, I'm talking to someone with a fairly thick east indian accent. 3 minutes later I had the new activation code punched in, and windows activated on my computer. I just had to reassure them that I hadn't installed the OS on more than one computer, and I told her that no, I hadn't, I'd just had some conflicts with windows, and had had to reformat and reinstall a few times. NO problem, just 5 minutes out of my life, and an 800 number too.