The issue is, and I believe it is carrier related is WM5/6 can have an option to support Multiple APNs.

If your carrier supports it, then WM can do both BBConnect and Exchange Push.

However that requirement is stored in the SIM card, so even if you change the required registry setting, on reboot it re-sets itself.

Enabling Multiple APN Support

I had to drop BBConnect for a 3rd party Exchange Push.

The only thing I have not been able to re-produce is the autoBCC that the BIS offers.

However, being able to type ahead my address book, able to do other things on the internet without losing my push email, and HTML email are reasons to move away from BBConnect at this time.