I've searched around for this specific problem but haven't seen any solution. Basically it seems the battery meter on my 3Gs is way off, but only when i connect my iPhone to my Mac. It'll start syncing, then whatever capacity the battery has, will disappear and bring up the 'connect to power screen' with the empty battery filling the screen. It then rakes up to 30 mins to go from that screen to turning back on. This has now happened 4 times, all when connecting to my Mac, connecting to a wall charger doesn't have this happen. I don't know what the issue is, but it means i can't sync my phone anymore as the sync cancels as the phone turns off!