The next version number of SmartPlaylist will jump from v2.8 to v4.0 to make clear that it is only for iOS 4 and NOT for iPhone OS 3.x - so don't upgrade to v4.0 unless you have updated your iPhone or iPod Touch to iOS 4 (to be released on 21. June 2010).
New feature: Yet more info about songs: skipCount, disc #, track #, releaseDate, albumArtist, isCompilation and comments.
For later versions it is planned to offer searching in comments (e.g. comments contain "wild!") and filtering by release date (e.g. release date before / in / after "1974").