I don't have a 3rd party BES, but I looked into it. T-Mobile will charge £25/m extra for this data plan. Coupled with the $20/m for a 3rd party BES, I decided I couldn't justify the price just for wireless sync.
Thanks for the reply, I finally managed to get through to someone who knew what they were talking about at T-Mobile and it's just as you say, possible but expensive. I'll weigh up the benefits and see if it's worth the cost.