Updated at 8:05 p.m. PDT: The first round of CNET Labs iPad heat testing is complete. Results are available in this blog by Senior Editor Eric Franklin. Though further tests will be conducted, the initial results find the new iPad to be only slightly warmer than the iPad 2, and in no way a danger to users.
Anecdotal reports have pointed out that the new iPad can get a little toasty. A statement from Apple asserts that the new iPad operates "well within our thermal specifications."
Personally, I've noticed that the new iPad runs slightly warmer than the iPad 2, especially when the screen is turned up to full brightness and you're playing one of the more intense 3D games (such as Infinity Blade). Having experienced far more extreme temperatures from the bottom of my laptop during HD video playback (something the iPad seems to handle quite well), I wasn't concerned about the iPad's warmth. I also found that the iPad cooled quickly when set aside.
In 2010, CNET reported on a similar complaint with the original iPad. Because of the new iPad's beefed-up processor, the increased energy demands of the improved screen, and larger battery, these same issues may be amplified. As with previous iPads, there's no internal fan to aid with cooling; instead, the aluminum casing is designed to dissipate heat.
For more information on the probable causes of the new iPad's temperature, read Brooke Crothers' analysis on CNET's Nanotech blog