I concur that I used to have to have an Ericsson T39 connected via Bluetooth for syncing. Whatever the reason is not the point. of the supposed 300 new innovations in Leopard, the removal of this elegant feature of Address Book is to be lamented.
In the history of Mac OS X development, the transparent menu bar (and menus for that matter, the lack of global Finder view control, the 'wobbly' stacks, and lack of Address Book Bluetooth support will surely go down as the biggest boo boos.
However, in the spirit of positivity, there is the quite wonderful Bluephone Elite application, which has just entered a Leopard friendly version 2.
The support level is quite astounding, as is the range of confirmed compatible devices.
The application takes a little getting used to, but now works (v2) quite cleverly like an email client, but for bluetooth devices.
It is at
http://mirasoftware.com/BPE2
Although I preferred the simplicity and elegance of the Address Book integration, this app does work much more efficiently. However, I actually expected the application to become an Address Book plug in, rather than Apple drop the Bluetooth support altogether. I am very surprised by this decision.