I'd also like an answer to this question. When I went to Digikey to buy chips for a board I'm designing, the 328 was end of life, so I purchased 25 of the 44 pin 324P devices.
My project is an alarm system for a vehicle that needs to be able to be 'rekeyed' (re-fobed actually) while out in the field. (250 trucks)
The unit sends a GPS location via Cellular to the home office once every 20 minutes. When it checks in, it will pick up a message if one is waiting and send it via serial to the 'duino which will then change the coding of the valid keyfob address.
As this is an 'automotive' application, I didn't see a problem using the 324P.