As much as I thought that the ipad was going to disappoint, it has really surprised me how far they have come already. The Gorrilaz album, The Fall, was all written and recorded on an ipad. Tascam has released a version of the Porta One 4 track cassette recorder for the ipad, the Portastudio.  Point is... it's 2011. Bitch.Korg seems to be leading the move to the ipad. They now have multiple music production apps available including the ielectribe:

The ims-20 is a version of korg's analog sytnth, the ms-20 (1978-1983). Nick Sharratt has done a great review on this app here, Synth project has taken these new apps to the next level by creating a synth controller. check it: