I've made some designs for a case. I'm working on the assumption that some people will not be tinkering with the hardware that much, that they will want something that keeps the R-Pi dust free and presentable to have in public (you might say 'wife friendly'). I have three different designs, all of which I'm thinking of getting 3D printed, probably in black but other colours too, possibly.
There is one that's a simple box, rounded corners.
There are two that have holes for fixing to a VESA mount on the back of a monitor or TV - seeing the announcement of XMBC earlier in the week, I don't think I'll be the only one hooking it up to my TV. The first has 4M holes at 75mm and 100mm spacings for monitors, the second has 8M holes at 200mm spacing for most TVs. I'm certain that the lightweight case and board will be securely fixed by two points, so there's no need to make a gigantic frame to put four bolts through.
Most of the cases I've seen so far seem to skirt around the SD card area. From the reference photos I've seen, I know I have to tidy that area up when I get the Pi board, but we'll be good to go in literally hours after it arrives. To those designing cases, don't forget that the SD slot is underneath the board, at most the depth I've indicated on my renders.
When I have a final design, I will release the .stl files on Thingiverse, but until then, I'm trying to gauge interest in this. How many people would be interested in one of these cases? I may set up a Kickstarter project to raise some funds, or make a shop on something like Etsy to sell them - would you buy one?