Sony announced on their PlayStation blog site that the have been working the past few months with Neversoft and Activision, Harmonix and Konami to develop a basic level of game play compatibility between different guitars and drum sets.
Current compatibility development is aimed at titles such as RockBand 2, Guitar Hero: World Tour and Rock Revolution. This means you would be able to use RockBand 2 peripherals with Guitar Hero: World Tour and Rock Revolution software or Guitar Hero: World Tour peripherals with RockBand 2 and Rock Revolutions software and so on...you get the idea.
Also, both RockBand 2 and Guitar Hero: World Tour will support Singstar microphones.
Sony also announced their intention to support compatibility between the current RockBand and Guitar Hero titles.
One key thing to note is that not all game play features will be cross compatible across all peripherals as each game has it's own unique features specific to its own controllers.
This is great news for fans of these music games and have been looking for flexibility in being able to share peripherals across the different titles and not to mention the potential to save money in not having to purchase an entire new set of peripherals to be able to play across the different games.