I bought a G-dec fender practice amp and it has an mp3 and headphone input. I can blend the vol of the mp3 and the guitar and practice through the head phones. you can easily seek and rewind on an ipod, pretty accurately. I find that to be the closest thing to what you are talking about. It was most useful trying to get my strat to sound like Europe 72, plus I can jam to Dark star with the boys.
The two things that has me wanting this over an amp that can take a line in is. 1) I have a small kid who like to sleep and doesn't like me jamming out, though god knows I've tried to break her. 2) it would be kind of nice to just need one cable, and not have to plug into to anything else if I just want to mess around with backing tracks or something else
ok number 3 I like the ability to record on it also so I can listen to a playback, and see how bad I suck
Another novel solution is the "iLift" iphone/ipod/ipad app....there is an app for that! Though I don't think it amplifies your instrument....it does everything else....and well for $1.99
Hey, Ya know what kind of practice tool I am looking for?... A "Video Looper" pedal. I like to play along with GD Videos, and yeah, I gotta pick up the remote and do the digital video rewind thing every time I want to see Jerrys fingers over and over a few times. I need a Video version of the Digitech JamMan looper. Anything like that out there?