I to have noticed this with mine when I'm running a highend game like AVP Evolution, RipTide GP, etc.
But it's not really that big of a deal for me as once I back out it cools down pretty quick.
When I've checked my battery temp it is usually between 99 - 104F, And cools back down quickly once I stop playing games and such.
To about 89 - 94F.
If your holding it against let's say your Lap that will cause it to become pretty warm at least more then normal because No air circulation behind it as well as your body heat.
Mine is in a Leather Case and gets along just fine.
So I'd say you have no worries.