A MacOSXHints reader has put
together an interesting set of Quicksilver scripts that will allow you to both easily tag your iTunes music as it is
playing, and create on-the-fly playlists based on these tags. The way they work is simple: while music is playing in
iTunes, you can use Quicksilver's text abilities to easily add tags to the music file's comments section. The beauty
here is that tags are prefixed with an asterisk (*), and they are appended to anything that might already be in the
song's comments field, preserving what you might already have added. The second script allows you to create an iTunes
playlist right from Quicksilver based on any of your tags.
I haven't gotten to test these out yet, but the download includes detailed instructions from the author's site.