I'm not sure if this is possible, though I couldn't find it anywhere - I think it would be amazing if you could somehow auto-convert songs to different bit-rates/format based on something like what playlist they are in.
For example, I have about 30,000 songs in my collection, and a good chunk of podcasts as well. I only have an 160GB iPod, so to carry all that around requires pretty small files. Smaller than I'd like, but I don't have the cash for a larger MP3 player yet. So what I'd like to be able to do is, for instance, auto-convert all my Podcasts to 64kbps mono MP3s if they aren't already, auto-convert all my 5 star songs to 256kpbs, and auto-convert a few select tracks to something like 320kpbs, and then have the rest of the songs go VBR at around 100kbps so I can still have EVERYTHING at my fingertips. I'd love to be able to do this by selecting different auto-covert rules by playlist, but obviously there could be some songs on multiple playlists - in which case I'd want to be able to specify the order of the filters so that it auto-converts to the highest quality of the matching filters. Maybe even just allowing the same selection filters for advanced auto-playlist creation, as for auto-conversion rules.
This would make having a large portable collection much more appealing to me.