Giving the developer the benefit of the doubt, one could speculate that this inconsistency will be dealt with in a downloadable patch. After all, the Xbox 360's Les Paul works just fine with the Xbox 360 variant of Rock Band -- this lack of peripheral support would only be rendered worse if it wasn't even consistent across systems. With no standalone Rock Band instruments available until 2008, the only way of completing the PS3 experience as things stand would be to purchase the $180 bundle again (as if finding one wasn't hard enough). Between your best friends and Harmonix, someone's gonna have to do you a favor.
We'll let you know when we hear more.