How to use an audio output to generate IR frequencies
This problem is really not that bad at all. You do not need to send the carrier frequency over the headphone jack. Instead, you should build the oscillator into your device a-la http://jap.hu/electronic/infrared.html (or just use a micro) and then you just need to switch it on and off based on a much slower signal from the headphone jack.

According to the internets, almost all remote control receivers expect a 38kHz carrier wave. You will need to turn it on and off at 2-4kHz. See also http://en.wikipedia.org/wiki/RC-5

If you are as clever as off-the-shelf iPhone headphone jack infrared transmitters then you can get your power from the audio signal, but if I were you I'd just use batteries.