Hi Philip, lots of questions and not many answers. Ok maybe I have some. Firstly my biggest problem with my Ipad controller app is it's not the top of the line 50 buck one (yes this is a virtual contoller with faders and the ability to contol effects). Which would allow me to use it with effects as well. Now I would normally jump in and pay my 50 bucks but the technolgy just keeps on keeping on. And what I mean by this is the new touch screen monitors and X2's ability to use these new monitors. My Ipad has a little screen, it would be nice to have a decent touch screen monitor to mix on. So you see my problem, I buy a 50 dollar app or I pay a few bucks more and get a touch screen monitor. On saying that when I do a little more work in the future I will buy the app and probably the monitor. And I will use both more.
This leads me to the next point which is these devices as instruments themselves, now in the studio I am a little dubious about using them because of how to get a decent sound out of the device. Yea you have a balanced 3.5 mm jack but I am not convinced that you are going to get the best sound. And why bother?? When you have very decent VST's that come with Sonar.
But on saying that where these devices will excell is live, and I caution you here I have yet to test this premise but I will at some point this year. But theoreticly on your Ipad or Iphone you have a myrid of synths and drum machines. My idea is to strip down my writing style to just an accoustic guitar and use the ipad via Reason (the software) as a basic drum and bass synth. You circumvent the need to take a much larger laptop on stage and also negate that risk. Of course you could use both and then the possibilties become even more amazing. Basicly you could switch between devices, much as a DJ does but only your a real musician and your playing the devices and an instrument. And this is where the technology is catching up, you can buy a midi input for your Ipad and or IK Multimedia have a contoller keyboard. And IK multimedia are bringing out a bluetooth midi foot controller and you could even, possibly use a Behringer FCB1010 foot controller as well. So I think you can start to see the possibilties, Yea??
As for the other questions, such as how you connect the devices, it's either via a wireless network, bluetooth or a input device designed by Apple. But mostly wireless and I could run you through how to set this up.
I'm not sure about REX players or samplers.
Finally android is a toy and to be avoided at all costs. Reason: there is not the infrustructre, controller keyboards and interfaces for android nor are there the apps. This may change but at the moment it's only Crapple.
Peace Ben