For a limited period, you can get iTunes tracks for just the cost of a standard txt message.
Text the word ROCK, LIVE, POP or ITUNES to 85100
This costs your normal network rate. You will get a text in return with a discount code you can enter in iTunes when buying a track, which will give you the track for free.
Expires September 30th. One use per mobile.
Thanks to Martin on MSE for the heads-up.
If anyone has a Sony Ericsson K810i (or similar) and their code contains a smiley face with the tongue stuck out then you should replace that with the letter P . Took me a while to figure that out!