Gotta be able to run a wintel executable, but if you can, the instant pricing feedback as you play with various design choices . . . wow! The app is a bit simple of course, but I'm given to understand it can import most any dxf file.
Err, or mebbe I'm hopelessly naive, there are already a sh$tload of similar services out there . . . ?
well, how about designing an injection molded ABS custom casing part for a . . . uhh, well, say a hacky PB or something . . .
Figured I'd better toss in some on-topic content.
I don't think there are too many out there who will do this at a consumer level.
Funnily enough, I just saw the link for this place a couple days ago at Starship Modeler. The only other place I know of is ToyBuilders.com who will cast in resin using stereolithography.
Pretty cool stuff for mod parts - and expensive!