[User] HOWTO: Use BlackBerry As Modem For Laptop!
But yeah, (as said above) you can only do GPRS...
What you could do, if your system supports it is wake on ring (if the system goes on standby; if not you just make the system wait for a call)... you would essentualy call the system with a normal voice call (using the phone physically) and hangup; the system could then dialup and make VNC (gui) & SSH (text), or whatever else you want available & dyndns could give you a "static" ip. Once thats up and running you would type in the ip address of the system and get in over GPRS. After use you would disconnect the remote system.
The issue here is that anyone calling the system would cause it to dialup... I haven't figured out how to do this (but I've heard its possible) is to have the system check the caller id; and only pickup and do this for your cellphone number. Then anyone can't make the system do that; and if you needed to get trough the line with voice you would just block your number so the computer doesn't respond.