It's an obvious statement, but when you're troubleshooting, always verify that your cable is good.
I've been wondering why my Treo wouldn't sync recently. First it was a rare problem, and now it's rare when it does sync. I didn't associate this at first, but sometimes it hasn't charged when I plug it in (oddly enough, it does sync some of those times). However, this seems like the cable is going bad.
Whew! That's an easier problem to solve!