I have the same issue with my SE W890i . Some SE phones do not work with BT DUN.

There is a different way to connect to the internet using the phone as a modem and that is via PAN,
If you have upgraded to OS2008 then which " version" have you upgraded too ??because PAN support seems to have vanished inthe latest 'diablo' release.

I have remained with the older 'chinook' release to maintain PAN support ( until an update is released). there seems to be a big demand for it so i'm sure it won't be long coming

Go to --> https://garage.maemo.org/projects/maemo-pan


If your phone pairs ok with your tablet then this should work ( once installed , just select PAN as a method of connection). IMHO it's works but still has some crashing and reboot issues ( for me at least i have to have a fresh reboot for it to work properly)