Making music on IOS

Since I have no experience of making music on the mac, I can't say much about that except to recommend a program called Reaper. It is the best! As far as iPhone apps go, there are tons. The ones you'll be most interested in are Thumbjam, Korg IK Oscilator, Garage Band, and, if Apple releases it, Alchemy Mobile. There is also an app that was just re-released to the app store called Songbird which turns your iPhone into an EWI, electronic wind instrument. When you play the sax, it really sounds like you're playing the sax!
Finally, for unconventional ways to make music, try Noatikl. It's going to get a huge accessibility boost soon, but that doesn't mean you can't use it now --I do all the time.
Finally, other people and I have podcasts on almost all these apps. Just search for them and the relevant podcasts will come up.
HTH!

I've been playing with apps like GarageBand and the Alchemy Mobile Studio for some time now. They're fun to use as touch instruments, but I've always wanted to be able to control them with my MIDI keyboard.
Recently, I purchased the iRig MIDI 2 interface from IK Multimedia. It has three ports on it: In, Through, and Out.
So far, I've just connected my keyboard via the MIDI Out port to the MIDI In port on the iRig (I believe I've got that right), and have been able to record some compositions more effectively in GarageBand, though I doubt you can save them as standard MIDI files to work on in a sequencer like Sonar, Logic, etc.
There are some apps from IK Multimedia that you can download for free like SampleTank, but I would personally avoid those as the accessibility isn't very good.
Overall, though, I'm glad I bought the iRig.
Other music apps I'd recommend include ThumbJam, DrumJam (which allows you to create all kinds of drum loops or play various percussion sounds via the touch screen or MIDI), and Korg's iKaossillator. The latter app doesn't support MIDI, but is still a lot of fun to play via the touch screen. All three are compatible with Voiceover as well.

Don't underestimaqe the touch screen as a playing medium. For example, think about steel guitars and how they sound. The guitarist might start with a C major chord, first inversion, slide up to the second inversion, then slide up to F second inversion. This is totally impossible to replicate using a keyboard unless you're going to create three tracks and plot out the pitch bends for each track necessary to create the corresponding sound, and you certainly will not be able to produce it live. Not so with the touch screen. Believe me when I tell you that I've fooled people more than once with my Thumbjam steel guitar, and that's on iPhone. With iPad, it's even easier to precisely slide through chords.
Next, take Songbird, by Songbird Ocarina LLC. With this app, you play your iPhone like a 4 hole ocarina. You have a three octave range, and can also bend notes either by tilting the phone up and down or by dragging one of the fingers on the right side off the hole, instead of just lifting. The volume of the instrument is breath controlled, so you literally have a wind instrument in your pocket. Unfortunately it can't send MIDI, but I'd pay the devs a good deal to allow it to do so.
In short, then, the touch screen is a powerful medium for the generation of music, and, as is the keyboard, invaluable. Therefore I suggest using both tools on a regular basis and excelling with each. Sooner or later, apps will come out where you strum your iDevice like a guitar with the right hand, and make the actual chords with the left, and who knows what else.