Here it is, a model of the Raspberry Pi B+ revision.
You can use this to ensure your case will have a good fit.
I took the measurements from the raspberrypi.org's blog, so, unless they made a mistake, this is as precise as can be as I also reused already made components for the HDMI, USB(s) and MicroSD card reader.
No guess work here.
V4 Update: As I test fit my case, I fixed the size and position of the USB and network components. We're talking sub-millimeter here, but still.
You can also see the case I made with this board design here: http://www.thingiverse.com/thing:423216
If you are looking for the Raspberry Pi A+ board, you can find it there: http://www.thingiverse.com/thing:566520