I've had this problem with all versions of OSX, but thought it might be rectified with the arrival of Jaguar, but no!
I connect to the internet from my iMac and iBook using the modem in my ABS (Graphite). Obviously I only want it to dial up when I require an Internet connection. However, it seems to want to be permanently dialed-up. It dials on startup, wake from sleep and just at random times. I've checked the obvious candidates - Software Update (set to manual) and Network Time (not selected) - but it still does what it wants.
I can set it so that TCP/IP connections aren't automatic but then it is a bit of a pain to have to manually connect every time I want to check email or use explorer.
Any ideas as to how I can prevent this annoying problem?