Every phone gets warm from time to time. The question is if your phone is getting warm ALL the time, or if it's getting HOT from time to time. In either case, I'd start to raise an eyebrow.
Warm is one thing, but my red L920 was getting unconfortably hot when simply browsing the net, funny though I could play Treasures of Montezuma or Mirror's Edge for a good hour and the phone would be warm at best. GPS, Browsing the web and especially charging the phone, I'd leave it on the wireless plate and when I pick it up it feels like it's going to melt my silcone case, sometimes it would get hot on its own, probably some background apps running or pulling my locations (GPS).
I ordered a replacement, I was 1 day shy of my 14 day buyers remorse but the store was out of stock so I'm just waiting for the unit to be mailed, hopefully the new unit doesn't have this issue, although my wife does have a black one and it seems fine, gets lukewarm at best with heavy browsing or anything else.