For what it's worth, I was having this problem too. I discovered that several things can turn on my iPod (2G/20GB) without much provocation:
* If I don't "lock" the keys, a random bump can press play.
* Even if the keys are locked, if I have the remote plugged in, it can also start the iPod playing if one of its buttons is pressed.
* If either/both the iPod and remote buttons are "locked," I've also had various items in my purse click the little switch to the "unlocked" position. Thereafter, the above points apply.
It seems I've encountered these situations both when my iPod is in its little case, and when it's just floating around in my purse.
Anyhow, it's a bummer whatever the cause - hope you figure it out.