The downside of greylisting, of course, is when you're waiting for a signup confirmation email. If you don't know where they're sending it from, you can't whitelist it. Plus that would require thinking about this beforehand. So you check the mail logs, and sure enough, there's the bounce. So you wait. And wait. Until, eventually, they retry. Hopefully.
edit: Interesting statistic: 2250 greylist rejections on rigel, just so far today.