PPM's won't be perfect, but they'd still be a huge improvement over the diary method. The PPM's are placed with the listeners for a longer period of time, so the numbers should be a bit more consistent too. It will be even more important for Arbitron to get sample right, however.
Still, I hear we're at least 3 years away and stations will be asked to pay nearly double for the PPM survey than they currently pay for diaries.
Trapper