IK Multimedia - makers of fine Guitar and Bass modeling software, among other things – have pre-order and pricing info available for their new iRig/Amplitube combo for iPhone, iPod Touch and iPad. This system requires their iRig dongle in conjunction with an app on your device. I have used both their Amplitube and SVX desktop modeling software before with good results. This mobile version looks to be a fun alternative for on-the-go jamming.
AmpliTube for iPhone is now available from the iTunes App store in 3 versions:
More info here