My brother-in-law swears by DRI WASH 'n GUARD waterless car wash. He uses it on his show cars and his daily driver. He claims it never scratches even on a dusty car (he has a dirt driveway). I haven't tried it, but I've seen him use it on a really filthy Subaru. Its quite expensive and comes with a special sprayer. There are some deals on E-bay though.

Here's a link to some of his pictures, I have seen him use it on the '56 Nomad too before he sold it. His show cars have been pretty nice, so I figure he must know what he's talking about.

http://community.webshots.com/user/bighorn56