In a statement on their Rock Band community forums, Harmonix claims that even though they have developed a patch that will allow PS3 Guitar Hero controllers to work with Rock Band, Sony is refusing to release it due to complaints from Activision. Here's what they had to say:

Hi Everyone,

As we have said in the past, Harmonix and MTV Games believe in an open standard philosophy of hardware and game compatibility. We think that there should be interoperability between music instrument controllers across all music games. This is clearly in the best interest of consumers, game developers and console manufacturers and will only help to grow the music game genre as well as inspire innovation and creativity.

Two weeks ago, Harmonix created a software patch for the Sony PLAYSTATION 3 version of Rock Band that allowed for guitar compatibility and support for third party peripherals, including enabling use of Activision's Guitar Hero III controller with Rock Band. The compatibility patch was submitted, approved and had been scheduled for release by Sony on Tuesday, December 4. Unfortunately, Activision objected to the compatibility patch's release. The patch remains with Sony, but we have been told that it will unfortunately not be released due to Activision's continued objection.

As is the case with the Microsoft Xbox360, we believe that Sony PLAYSTATION 3 users should be able to use the peripheral of their choice with Rock Band. We sincerely hope that Activision will reverse its decision and allow release of the compatibility patch and further, that Activision will allow Guitar Hero III to support Rock Band guitar controllers as well. We welcome all third party developers who wish to support our controllers and will provide any required support in order for them to do so.

We believe that when consumers have choice, everyone wins. Harmonix and MTV Games hope that Activision and others will also adopt this philosophy.

Sincerely,
The Harmonix Team