it takes time for the phone to connect to the bluetooth again.
i use my 2 phones in my car. factory installed BT system in verna.
1. xperia arc - the phone connects but don't resume the playback.
i asked a similar question here as well - [Q] Automatically Play Songs When Connected To Car Bluetooth.. - xda-developers
2. nokia n 95 - this works great. after i start the car the connection is lost but the phone re-connects and even resume the song
so i guess it depends on phone to phone.