There is a huge number of music production apps for the iPhone and iPad, among them Apple's own Garage Band. These apps have generally been great fun for use just on the device, but using your own instruments as opposed to the software instruments has always been a pain.
That is, until today. IK Multimedia, who develop the popular guitar app AmpliTube, have just released their iRig Pro, a minature audio interface designed to solve all your audio/MIDI problems. The new device supports almost all types of audio input, with the ability to connect with either a standard jack (like an electric guitar) or an XLR input, through which you can connect microphones, keyboards and a whole host of other instruments. You can even adjust the input gain to make all the levels right before you record.
For many musicians (including myself) who have been itching to get the most out of the fantastic potential in iOS music apps, this is the answer to your prayers.