Guitar Rising was first to announce a real guitar game back in 2008 but never released, presumably because of problems with the polyphonic pitch detection.
The first real guitar game released was LittleBigStar, back in 2009. LittleBigStar supported a wide range of instruments, including guitar and bass, and loaded mp3s and standard tablatures in different open formats. It had a good momentum and indie developers made different kinds of musicgames, which they called MusicWare, but it was closed down two years ago. By those measures RockSmith is hardly new...
The LittleBigStar team decided to go commercial, presumably because they had success cracking the polyphonic pitch detection nut. They released Offbeat guitarist which is freeware, support open formats and works great.
In 2009 Disney claimed to have found the holy grail of music gaming: Disney Star Guitarist but it was never released.
In 2010 Rise of the SixString was released with a guitar-controller hybrid.
Holiday 2010, Harmonix showed RockBand 3 pro-mode with the Squier Strat Controller. It went for sale in BestBuy stores in March 2011.
Holiday 2011, UbiSoft claim to have found the big new thing...