Thanks to the help from this group I was successfully able to retrofit bluetooth capability into my MY2004 e60 530i with a production month of 02/04.

My vehicle was already equipped with the BMW Assist option so all I needed to replace was the existing TCU module w/ a BT-enabled TCU module (part no 84109126660). I was able to buy a new TCU module from a fellow e60.net member for $325. My local BMW dealer performed the TCU module swap and TCU reprogramming for 1.5 hours of labor whic came to about $175 broken down as 1/2 hour to install and 1 hour to recode.

I was able to get my Treo 650p (verizon) smartphone paired to the car via handset mode no problems. Not every feature works, but I can dial, access my phone's quick address book (but not the vzwsync address book), redial and end calls. I am very happy to be able to check voicemail on my way home through the car's audio system!