Magnus,

You're correct, they are limited to one search type at a time. If you are thinking about Allmusic.com integration and are conversant in Perl, check out the WWW::AllMusicGude Perl module. You could borrow from some of the logic there which does the dirty work of initiating the search, parsing results, etc. The AllMusic folks don't exactly make it easy to parse things out.

As for searching on multiple fields, a program could theoretically initiate both an artist and a title search and then find the union of those two... But that's a hassle.