Are you running Windows 7? I have two laptops, the one with a XP on it tethers with 8900 just fine, With Windows 7 I can't resolve any IPs (DNS servers not responding) In both cases tether via bluetooth, same modem config just different OS.
If I set up Windows 7 laptop as a stereo device via bluetooth and start playback of music on 8900 with a laptop acting as a speaker for blackberry (creating a traffic on bluetooth link) then DNS issues disappears, in other words I have to listen to the music in order to get Internet traffic flow