Just got my moto x yesterday and I noticed that while browsing it starts to get quite warm. Also did some syncing of files with my computer and it started getting warm again.
It got almost hot last night while browsing until I rebooted. Still runs a bit warm while syncing or downloading. I'll see how it goes over the next few days before I decide to possibly return for a new one.
I noticed mine was hot (not warm) today while it was in my pocket. I'm looking at my running apps and I don't see anything strange. After a reboot, everything was fine.