I guess my iPad wasn't small enough for my music noodling, so a new addition to my collection is an iPod touch 4th gen and the Akai Synthstation 25. Some of my music software runs well on the iPod (NanoStudio, Xenon Studio, Moog Filtatron) but a few others I had to buy again (Nlog Midi is a nice mini version of NLog Pro synthesizer with a few less oscillators).
I also picked up the synthstation software from Akai which lets you play with 3 synths and a drum machine all of $2. Overall, however, NanoStudio is the best product that supports the synthstation.
NanoStudio runs flawlessly, uses some of the buttons to easily switch from window to window and sounds great. The synthstation is a bit different than the MPK mini that I also use in that it has no drumpads or knobs but does include the pitch bend wheel and mod wheel. It also can work with my iMac or iPad if I need the wheel features. I will probably almost never plug in my old Edirol PCR-30 now.
I have used the iPod touch by itself for a few NanoStudio pieces and it is actually not that terrible, but plugged into the Akia synthstation its a pleasure to use.
It doesn't have speakers so to hear your sound you have to use headphones and it is not compatible with every iPod music app but it is a very nice addition to my music gear.
It is a bit of a pain moving project files around for NanoStudio right now since you have to run NanoSync to move files from the portable devices to the computer and back again. Perhaps the new iCloud product from Apple will be of use in future versions - I would love to seamlessly work on projects from any device.