Plans

Google recently bought a small company with a very nice 3D design program called SketchUp. There's a free version available, and though it has a few quirks, it is ridiculously easy to use, especially when compared to other CAD programs.

We started with a few sketches, which you can see on the right here. After finding a 27" TV off Craigslist, we were able to make real measurements and mocked up the one on the left.
Further tweaking was applied, and we ended up with something similar to what you see here:

In reality the final design is slightly different - specifically, we raised the control panel about 3", and the marquee about 3" on top of that. The final height was six feet in total.

You can download the plans themselves from here:
http://crummynz.shackspace.com/pics/cabinet2/cabinet_plans_2.skb (Remember, you need SketchUp to read these).

Our other planning has been around the control panel. Our most popular feedback after our initial "bartop" cabinet was for two player support, so that was definitely in. For a while we tossed up between a trackball or a lightgun, but went for the trackball as we'd get greater game support with it. You can see an approximate layout here:

The top three buttons are player 1 start, administration (or pause?), and player 2 start. The middle circle is the trackball.