Harmonix has recently revealed that Rock Band 2 will support 3rd party peripherals.  3rd party cymbals can be attached to the official Harmonix kit to help simulate a real drum set.  There will also be a $299 Ion Drum kit that contains 4 high quality, arrangeable drum pads, and 3 "cymbols" corresponding to the colors yellow, blue, and green on the drums. 

Naturally, you'd assume the cymbals would be supported in the game.  In both the Guitar Hero and Rock Band series(post GH 1), the guitar and bass instruments include "Hammer On and Pull Off" notes.  These mean that you don't have to strum these notes, assuming you press the button at the correct time.  Annotated a different note on the chart(white ring on GH, smaller note on RB).  A natural progression for RB2 drums would be some sort of variation in the note on the chart, that says "hey, you can hit the cymbal if you want".

So far, signs point to this not being included in the game. Harmonix developer Johnlok himself stated that if you listen to the song, you will eventually figure out for yourself the appropriate pad to hit, and annotation isn't required. http://www.rockband.com/forums/showthread.php?t=65092&page=8

Guitar Hero: World Tour will include a drumset with 3 pads, and 2 cymbals.  It has also been stated that the game is compatibile with Rock Band peripherals, and that the note charts for songs will actually change to fit with your drum set.  So why can't Harmonix change the note charts of songs to fit their own peripherals?