I'm a music geek, I just chose to carry a dedicated music player.

Yeah I wasn't talking about that kind of music geek :)

There are some pretty great instrument apps available for the iphone, like the ikaossilator by korg, a tonne of great drum machines, loop apps, synths etc. And mixed with the Audiobus app that lets you route the sound of one app into the input of another (as long as both apps support Audiobus which most seem to do), the possibilities for creating music entirely on your phone are limitless

The issue isn't about having your phone play music, it's about you playing music on your phone, in some cases with friends who play on their own phones.

Ah, for that I have a workstation. But as my guitar playing is very poor it doesn't help a great deal.

It seems you're talking about two different things still. If you have an iOS device, you can connect an electric guitar to it via a dedicated (but relatively inexpensive) interface and have it sound like a reasonably amp plus some effect processors. It's really great for mobile musicians. I don't even mention Garageband and other apps which basically allow you to compose quite decent music on the go.