OK figured it out, after trying all the various recommended settings here for those of us without tether plans.
I checked all the settings on the BB handset in Options>TCP/IP. I then copied those over into the network settings on my Mac. Presto.
telephone number: *99***2# (also worked with *99***1#)
username: WAP(AT)CINGULARGPRS.COM (won't let me post email address yet)
then in the advanced screen
used both the Sierra and RIM modem selections, and the RIM setting seemed every so slightly faster according to speedtest.net. That said, the differences aren't out of the range of uncertainty so I wouldn't go so far as to categorically say one is faster than the other.
It seems as though the key to getting this to work properly is ensuring that the info in (Mac) System Preferences>Network>RIM connected device are the same as those in (BB) Options>TCP/IP.