It looks like AOL is bouncing emails sent from our server. I have two public Drupal-based sites and I'm now getting bounce messages for everyone's signup or new password emails.
They have been intermittently blocking mail coming from our shared hosting servers for the past day or two. The messages are only deferred and they should be delivered fairly soon.
AOL sux... If you use it, dump it and get a real ISP. If your friends use it, dump them and find better friends!
I don't use AOL but many of the people registering at my site do, and they never get their password mailed to them.