I also had this although it was every ~5 mins. I'd already used the fake server trick to try and make sure ActiveSync wasn't connecting in the background, but I couldn't fix it and I've just reinstalled 2003se. I don't have email set up on it, so it shouldn't have been background checking for email.
My thread is
here which has a couple of suggestions for stopping it. I didn't implement either of them as I just want something I can plug in and go. I'm not that attached to WM5 really, so I've moved back to 2003se rather than using the cumbersome workaround suggested by someone. The "turn the PDA off straight away" thing did work, until I undocked and redocked it, then it started turning on at random again.