Desks aren't what they used to be; with a notebook computer you can work anywhere, but what is most important is that you get the ergonomics right. Some people, like TreeHugger founder Graham Hill, swear by stand-up desks as being better for your back and for keeping you alert. There are a couple of fancy adjustable ones around, like Stilvoll's Amazing Transformer Stand-up Desk we saw at ICFF last year.
You may also not have a lot of space, and may have to share your office with other uses. This calls for Transformer furniture, where the desk can disappear or change into other functions. We have covered lots of this on Treehugger; start with Transformer Furniture Goes Mainstream.
You might also consider a used desk; the quality is often far better than what they make today, there is not outgassing of formaldehyde or any VOCs because they all outgassed years ago, and they hold their value. Planet Green food writer Kelly sits at a 1954 George Nelson desk that was considered junk a few years ago but is now a collectors item.
But if you do buy new, try and get decent quality, no VOC laden particle board (which knocks out the cheap Staples and IKEA stuff) and consider flatpack designs that you can easily move when you have to. Collin did a good roundup on TreeHugger: BuyGreen: Desks and Workstations
Read the next page "Where do you sit?"