i am currently tethered to a blackberry curve without the need of additional codes.
my set up:
at&t blackberry curve
nokia N810

first be sure the bluetooth is enabled and both devices recognize eachother (obviously)
then in your connection settings add a connection.
choose packet data call and choose gsm (for att)
now in the next page set up the connection apn and password.
apn=blackberry.net
user name=wap.cingulargprs.com
password=CINGULAR1
i didnt change the *99#

after clicking next i clicked advanced>other tab>and unchecked use ppp compression

now your done, just make sure your N810 phone settings in the control panel have the blackberry as a phone devide and you should be set.