I don't know if this has been discussed (I did search mind you) but I was able to create Genius Playlists with iTunes Match enabled on 5.0.1 (9A406) I believe this was a feature added with the arrival of 5.1. Now, there is a catch to create these Genius Playlists... I found that while listening to the song of your choice, tap the album art to show the iPod controls. Now typically on 5.0.1, you won't see the Genius icon (to my knowledge) Keep the controls showing and close out of the app. (go to home and kill the iPod app) Open the iPod app again and notice the Genius icon is there! It doesn't go away until you get rid of the iPod controls again or navigate away from the song. With this, you're able to create a Genius Playlist from the song of your choice!!! It allows you to save the playlist and will even show up as a Genius Playlist in your "playlist" tab in the iPod app.
Hope this is a new discovery and not "old news".
Hope this is a new discovery and not "old news".