There was a poll a long time ago about new materials and (cast) aluminium was one of the options in that poll. I would be really interested in this material for functional parts. I know that machining would be an option for many parts, but I think having the parts frinted (in wax and the cast) would have its upsides as well. At least one being that I wouldn't need to learn another 3D app to produce solid model suitable for machining (currenty using Sketchup). I've had propellers for a scale model ship cast in brass over at "that other 3d printing service" and they're the best propellers I've ever had.
I'd imagine since we're already able to cast silver, that the process of casting aluminium wouldn't be that different, but I'd love to hear some official comments on casting aluminium. Personally I think it would be a great addition targeting scale modellers.
I think Shapeways would do well with offering a high resolution metal other than silver. There are so many people that have an interest in mechanical parts! I too have used the services of other 3D printing service bureaus for high resolution prints in metals other than silver.
I'd like to see Shapeways offer this service cuz I know they'll do it cheaper than anyone out there.