I think the best available information on temperature is in the faq at
"What is the usable temperature range?
The Raspberry Pi is built from commercial chips which are qualified to different temperature ranges; the LAN9512 is specified by the manufacturers being qualified from 0°C to 70°C, while the AP is qualified from -40°C to 85°C. You may well find that the board will work outside those temperatures, but we’re not qualifying the board itself to these extremes."
I am working for a company doing real outdoor stuff. You can expect such electronic components to be 2 or 3 times the price of normal commercial ones... so don't expect a real outdoor board under about $100 ;-)
But if you build a really good case and make sure it doesn't get wet (careful: normal air at room temperature gets very wet air at 20° less and will suddenly soak your print! You have to get the air inside the case very dry!) and it doesn't overheat at the other side you can probably make it work well (I just wonder if it was an idea to bury it... temperature will be more constant below surface... but I never tried that out ;-) It probably helps against overheating that the CPU isn't the fastest one you could think of (something many people have heavily criticised - but it does always depend on what you want to do with it, doesn't it?).
I was looking for any info on the recommended minimum and maximum operating temperatures for RPi. I couldn't find it listed anywhere. I'd like to use this in a rainproof box outdoors, but it gets pretty hot outside, so I need something that has an acceptable temperature range.