My iBook now intermittently tries to dial up my ISP since I changed Safari preferences to auto dial when opened. Even without Safari open it tries to connect! Nothing is set to auto start in my Account preferences.
Do you have "Set Date and Time Automatically" ("Use Network Time Server" in older OS versions), checked in the Date and Time pref pane? Your iBook may be trying to dial up the time. There are a lot of apps and utilities that do a quick hit to a net site for one purpose or another. If you are using Auto Dial, you may be dialing up more often and for longer than you care to.