Here's another interesting new iPad music app, Retronym's Synth For iPad.
Features:
Considering its $.99 price and feature set, it's at the introductory end of iPad music apps.
Nevertheless, apps like this are going to put synthesizers into the hands of tens of thousands of new users - which could end up being something amazing.
Link:
James LewinDevelopments for Korg's instrument have been slow but promising.