The only thing that rapidshare could actually get anyone prosecuted for is something like child abuse photos or something of that nature. It'd have to be content-related, and the content would have to be *really* illegal (child abuse, terrorist plots, etc - not something like, say, how to make home-made drug paraphernalia.)
They could go after you civilly, but it'd be a cool case to bring to court - someone writing software isn't actually accessing rapidshare's servers, so a case against them would be nigh impossible to actually win, especially if the software author left it to end-users to type in "rapidshare.com" or whatever.