According to Reuters, guitar manufacturer Gibson has apparently written to Activision claiming that Guitar Hero infringes one of its patents. From the report:

Gibson said the games, in which players press buttons on a guitar-shaped controller in time with notes on a TV screen, violates a 1999 patent for technology to simulate a musical performance.

Puzzlingly though, Activision does have a license to include the guitars in its game, and to model the guitar controller around Gibson's products. I'm no legal expert (despite my ongoing court claim against a fraudulent eBay seller, but that's another story), but it seems Gibson is happy for the guitars to be digitised and for likenesses to be constructed, but not for those likenesses to be used to control the game:

A copy of Gibson's patent included in the court filing showed a method for simulating a live performance using a musical instrument, a 3D headset with stereo speakers, and a pre-recorded concert.

Gibson's law firm wrote in the letter:

Based on our preliminary analysis, the 'Guitar Hero' software (including any expansion packs) and the guitar controller provided by Activision being used as a musical instrument (packaged with the software or sold standalone) are covered by the ... patent... Gibson requests that Activision obtain a license under Gibson's ... patent or halt sales of any version of the 'Guitar Hero' game software."

Activision has apparently since filed a lawsuit against Gibson asking the U.S. District Court for Central California to declare the patent invalid - the videogame publisher, quite reasonably it would appear, is wondering why Gibson waited three years after the first Guitar Hero game to make its claim; three years in which (and this is possibly not entirely coincidental to Gibson's action) the series has racked up over $1 billion in sales.