I entirely agree with the need to be able to do this, and this is the main feature keeping me from making the migration from iTunes.
To respond to MrSinatra: having to do all of that manually completely defeats the purpose of having a smart playlist set up. I listen to a lot of podcasts, as well has have a music collection that is far too large to fit on my phone. It is too much effort for me to have to pull each of my new podcasts over into a playlist when they will all update at different intervals and I may be listening and catching up on them at different speeds. The benefit of a smart playlist is that I can say I want any podcasts with a play count of < 1 to show up in this playlist. Then I never have to worry about it again. Play counts get updated when I sync, those podcasts are removed from the playlist, and any new podcasts are then synced to my phone. It seamlessly groups and maintains the podcasts I want to listen to.
I also make similar playlists with my music, keeping only a subset of the stuff I have set at a certain rating, based on what I haven't heard recently. So I have a playlist of 3 stars that is steadily rotated so I'm not listening to the same stuff until it gets stale. I'm able to keep several different playlists updated and changing without having to hand cherry-pick everything that goes into them.
I realize it's a matter of laziness, but it's a convenience I've grown accustomed to with iTunes, and it would take a lot for me to give it up. This feature is the only reason I would ever get around to updating my playlists and listening to new stuff on a regular basis.