Rhinowing may have more experience with the iMic than I do, so I would not discount his advice, but I would look at the bigger picture:
Even if this iMic thing is better than I think/suspect it is, to get even a vaguely tolerable recording, you need to:
- go through heavy pat down
- rather than enjoy the show with your friends, sit/stand stock still throughout the show, presumably holding or otherwise fixing the iphone and mic in position to pick up the sound properly. This is a lot easier said than done. And I promise you, the show will be a LOT more stressful and a lot less enjoyable while doing it.
- not get thrown out/caught once inside
The iPhone is not a recording device. It is not designed to record music. The decent portable recorders with built-in mics like the Tascam DR-07/08 and Zoom H2 are light years better (they start at around $100-150) than an iPhone will ever be, and IMHO the recordings made with the internals on those decks still sound like garbage. Not to mention that using those internals, again, requires holding the recorder physically, making you kind of obvious. You can put the deck in a shirt pocket, but then, if it uses cardiod mics, you are aiming the mics at the ceiling, and you have it at a well below optimum height.
I don't want to discourage you from taping at all - it's a great hobby (though see item #2 up there about standing still/enjoying the show... it is a tradeoff). But if you want to get into, don't try something like recording a well-known and anti-taping band with equipment that even in skilled hands will sound poor. Just go and have fun.
If you still want to get into taping, start reading this board. For ~200 you can get some gear that is stealthy, portable and will get you recordings you actually want to listen to.