DVForge have come up with a nifty little gadget to let you play your guitar along with your iPod.
To build a small guitar amplifier module that plugs into the top of any Apple iPod music player, that lets a guitar or bass player practice along with songs on the iPod, through the iPod’s own earbuds. Design goal
See the link above for more details.
And at $30 it sounds the trick, though you can’t slow the tracks down to help you along, who knows maybe an idea for the future.