I agree with meerkitty. Or something similar sounding, like what you sometimes hear in industrialized songs.
I'm tempted to make a long post on this, but I haven't got the time right now. Because rock isen't just some tool to split up music now a days, it's also a genre with a long, long history, which plays a part in what we see it as today and also explains why everyone has so many different conceptions of it.
Rock has always (at least untill recently, 90s) had this rebellious or outrageous thing to it. Kiss with their costues and make-up, rolling stone, were even more rebellious and rocking than the pop oriented beatles and were and liked by the "bad asses". Smae goes for heavy metal, punk, grunge... And as long as it's kept that way, somewhat not mainstream and heavily tied to a fashion or fad, it's not that hard to categorize it. THe problem surfaces whan you get stuff like "bratz", stream lined produced idol "rock", and particularly in jpop where virtually every combination in sound avaliable is tried out by some artist.
Also, pop has the wondeful ability to turn elements from other forms of music into catchy and somewhat mainstreamish likeable stuff. It's like a sponge that absorbs everything, and when you squeeze it, you don't know what comes out.
We had a discussion on how much "guitar" that had to be present, and how evident it should be in order to call it rock... But it's hard to draw a line. Examples are biyuuden's
kacchoize! Japan vs f.ex
ayu's GAME. Both use a lot of el guitars all along accompanying all along, both have catchlines in el guitar. Both songs also have very apparant synth elements and no acoustic drums. Still I don't think a lot of people would consider biyuuden's song rock, but a lot of people on this forum seem to think GAME is. ... Kacchoize ! Japan has a totally different beat to it, it's bouncy and the percussion is like in a lot of dance / pop songs, the percussion used in GAME is very different. An important thing of course is also that the image tied up to the song. AYu's wearing latex and a total scary rock band image in the PV, while biyuuden is having really girly cute outfits and doing "some silly dance".