That's what
AOL has claimed it has noticed; spam messages caught in filters have declined 20%, and complaints by users have almost halved. Besides,
AOL is blocking spammers' sites (via
Slashdot), which might be just another step in an arms race.
I haven't seen that decline in my own mailboxes, but, if AOL says it, it must be true. Or maybe people have grown so used to it, they don't even notice.