Since the introduction of the Apple iPhone and iPod Touch, a lot of music-related apps have become available. Despite it's limited screensize, as well as other hardware and processor speed limitations, some of these apps are quite impressive.
In Apple's appstore, the most important types of music applications are in the following area's:
1) Personal radio (e.g. Pandora radio, Last.fm)
2) Radio broadcasting (e.g. iHeart radio)
3) Musical instruments, drum computers, synths (e.g. Ocarina)
4) Music tools, scratching, recognition, lyrics, chord voicings, tuner (e.g. Shazam)
5) Artist (e.g. Lady Gaga)
6) Recording/Sequencing (e.g. FourTrack)
7) Controller (e.g. TouchOSC)
With the recent introduction of the iPad, some of the above music app categories might benefit a little extra from the enlarged screensize, in particular the controller section. Of course it is still early days to see how this all develops but due to its large touch sensitive interface there is definitely a future for tablet PCs in music!
Createdigitalmusic.com gives an excellent overview of music app developments on iPad.