For the people that haven't played the game and are wondering how it can be fun, here's my best description:
The game field looks like a guitar neck, and colored "notes" fall from the top, tetris-like in 5 slots, left to right. As the note hits the bottom of the screen, you need press the same colored button on the neck of your guitar, and the strummer bar, which is a "flipper" that can be strummed up or down, which represents the strings. That's pretty simple.
The easy levels are VERY easy for a beginner to video gaming, guitars, or both, to play immediately and have fun. The songs are built to get more challenging as you progress. Most of the music are (usually) very good covers that have been recreated for the game. Some bands have come back to rerecord their songs for the game. This is necessary because if you play the wrong note, you get a "blonk" sound of the frets, instead of the guitar track. It really gives you the feel of playing a guitar, or at least that you are really helping to create music, anyways.
I highly recommend that every one play it at least once. I saw it in the stores when it came out, and thought it looked stupid. A friend brought it over to a party, and everyone, including myself, was consumed with it. The Wii is the only thing that's shelved GH from the most recent parties.
I find it interesting that so many games have been released lately that have crossover appeal to non-gamers.
I'll be curious to see if GH3 for the PC receives any patches to run more smoothly, as I've heard that it has ridiculous system reqs...