Fuck yes.
Check this out VIDEO LINK
15 bucks
I have looked into just about every one of these things, but decided I didn't need them. Yes, they look cool on the surface, but do you really need to create entire songs on the go? If you do, then there are a few options and this is one of them. However, there is nothing that gets even close to great sounds. iGOG is pretty good, as it is based around how hard to hit the screen (just like dynamic tracking in Drumagog) and you can load MIDI files into it. As a guitar player, this works great, because I also use a Vox Headphone amp (those little ones that have an AUX in, i think they are called the AmPlug) so then I can put on a groove/beat and then work on riffs with decent guitar sound all in my headphones
I forgot the other awesome program for this, which is Octopod. Look up Octopod also. Octopod comes with built in drum grooves of all types at different BPMs, so for working out riffs it is great. I also built a cable that goes from my Vox headphone amp to the MIC in of the iPhone, so that I can also record quick ideas on the fly so I don't forget them.
I don't do full production on my iphone, it is just a waste of time, but it is an invaluable tool for on the road or away from home work. Ever since I got all these tools down, I have been writing enough material for 3 bands, so I decided to start a sideproject.
Trent