I can't speak for the SDHC card problems but it is only natural that the computer is going to get hot seeing as it has no fans at all.
Here are a few tips/info I can share from experience.
1. The device gets hottest when on a surface that blocks all vents on the bottom of the computer. Like either a bed/fabric cover furniture or carpeted floor would be the worst because it forms a seal around the bottom not letting any ambient air in. I noticed the plastic tray in an Airplane also is not a very good place for very long periods of time.
2. I often found that placing the Mini either in your lap with space between your legs or slightly elevated in the rear helped considerably with keeping the temperature lower.
3. Any wind be it through an open window or if you are outside or even the air vent on a airplane help with keeping it cool. I was on a flight with my Mini9 on the tray and it felt a little warm so I placed it on my lap with the air vent blowing towards it and within minutes it returned to practically a neutral temp.
4. You can expect this puppy to get warm, but I wouldn't worry about damage so long as it doesn't get so hot that you can't touch it for long periods or if it stays hot for several hours.
I am trying to make a homemade aluminum heatsink for the HDD and Wireless card in mine so I can update with that progress later