I use the Spare Parts app to check out my battery information, and it gives a temperature there. I recommend that app, just cuz it'll give you lots of information regarding battery usage as well.
Right now my temperature is at 39.8C, and it's been charging. I will use it and then update this post when I feel that it gets hotter.
UPDATE
After playing games on it for about 20 minutes with 3G turned on, the temperature came up to 44.3C. That isn't all that bad, considering that it's a mini computer inside without any fans. My computer is running at 34.6C right now at just web browsing, for some kind of comparison.