I made an app that can hackishly control a MakerBot! Code is here. Clone via Git, open and compile with XCode 4.
I say "hackishly" because it only works on the X-axis right now and it only supports absolute coordinates. Eventually, I want this to become a full ReplicatorG implementation so you won't need a computer to control a MakerBot.
You need a Redpark serial cable and EITHER a provisioned iOS device and developer program membership OR a jailbroken iOS device and modified XCode. I have the latter.
If you want to help the development, feel free to fork the repository and do any dev work you want! This is a great resource to use. I really do need help with this because my coding skills are less than adequate.