I'm guessing you're on dial-up. There should be a check box under PPP options in System Preferences that says "prompt to stay connected every [10] minutes" See if that is checked off.
Actually DSL through Earthlink... Also it seems to happen every 15 minutes... and there is no options to turn it off in either "Internet Connect" or "Network Preferences"
I actually didn't see any options on the "AirPort" or "Ethernet" Connections, however I looked into the "Internal Modem" and "PPP Settings/Options" was on for 15 minutes...