My Dimension XPSR400 has custom power connectors from the power supply (it's not the ATX standard type, but there are directions on the net on how to convert that PS into standard ATX if you wanted to switch the mobo) to the mobo and also the front buttons are not standard. They too have a special connection to the motherboard, in fact..those buttons have a little board with some chips on it. Dell definitely gets custom made cases, but they are VERY nice. This XPSR400 only has 1 exhaust fan, and it's a p2-400!! there isn't even a fan on the p2 chip! The air flows extremely well through their cases, and they even have the HD situated vertical in front of the front intake vents..so that gets good cooling too. They make awesome cases, and I'm positive that they do design in-house at dell. I've read some articles before that talked about how the big name companies take everything into consideration when building a system (case quietness, cooling, power, etc.)
In other words, you will only be able to pick a case up at ebay, or if someone is junking their dell. Good luck!