The ThinkOutside Stowaway Bluetooth keyboard works pretty well. It is a nice combination with the 770 -- the nokia sits in its stand like it was made for it.
However, be aware that there is no number row -- so you will need to use the two colored function keys to get numbers and some other characters. Also the forward slash (/) is to the right of the right_shift key instead of to the left as on keyboards I use the most -- so I sometimes hit the wrong key.