iOS has support for something called XPC. IIRC, it's how the mail composer, etc is invoked by apps (behind the scenes). I'd be very surprised if the API isn't made public in the next version of iOS, which would help facilitate some of these things - it would at least allow easier app-to-app integration.
More info: http://oleb.net/blog/2012/10/remote-view-controllers-in-ios-...