In the last several weeks there were few things happened that really made music production process on the iPad much more complete.
First GarageBand came with Audiobus support and on the same week Audiobus achieved 100 apps that support technology. Which means that Audiobus now is the standard technology in iOS approved by Apple.
Then Auria was updated with time stretch and sidechain functions (there's no side chain compression yet - but ProC will support Auria sidechain in the next release).
After SampleTank (with great sounds library) and FL Studio (not a great as a DAW but very good sound library) were updated with Audiobus. So now it can be controlled with virtual midi and recorded in other apps via Audiobus. IK Multimedia (developer of SampleTank) also updated their effects app - AmpliTube.
Few days ago NanoStudio announced to implement AudioBus.
And finally iMini - iPad reincarnation of MiniMoog was updated with Audiobus and Virtual Midi support.
With so many great apps like SampleTank and iMini and DAWs like Cubasis, Auria and BeatMaker 2 the music production environment on iPad is becoming more and more advanced each week.