My iPhone4 has not been connecting to my btb200 automicatically either while my 3gs did. ! I have been struggling with this since I got an iPhone4 and finally figured it out today.
I do not have an AVIC-D3. Instead I have a group one pioneer head unit that also uses the btb200. The key is that non-AVIC head units can only regisater one phone at a time.
How to autoconnect:
I connected the phone to the bluetooth from the iPhone's Bluetooth menu. Then I put my pioneer unit on Bluethooth for source and TEL for band. THen I cycled on the function key until I was in FUNC2. Then I pressed the up arrow/button on the head unit. A few seconds later... phone was Registered!
I turned the car off an back on... it auto connected. This is the best thing ever.