Haha now that I have your attention... just kidding...
I'm currently running Fedora Core 1 (previously RH9) and have just got a new external modem (yay no more WinModem crap). I noticed that my modem would randomly disconnect and thought that my new modem wouldn't - but it's been happening all night
Is there a setting somewhere that says that if it's inactive for a certain amount of time to kill it or something?