Here is my first shot at a Raspberry Pi case, I took the mechanical drawings from the Pi website and used it to calculate the positions of the mounting holes and overlapping items. The Pi is held into the case with some screws (that aren't really necessary) I put some nice rounded corners onto the top and the whole top cover just clips into place so no screws.
The HDMI holes could be smaller and the power connector hole could be smaller too. It doesn't have GPIO slots yet but I'll add those as and when I need them. Now to get playing with RetroPie