This may sound stupid, but need to ask anyway I was wondering when you mount radiators on the bottom of the case, so the fans will blow the hot air out straight on your desk - will that defect your desk after, lets say, a year of blowing hot air? Something like a fade colors (wooden desk)?
What kind of desk/wood? Edit: I'd be more worried about the weight of the case rather than the heat TBH.
I'd be more concerned with the draft of air across the desk - could cause problems if you put any sheets of paper on there at all...
Wood has much more problems with humidity than with temperature... And as Krikkit said, even if you overclocked a bi-phantom with 4 SLI GPUs, the temperature wouldn't be high enough to affect the wood...