I don't think you're required to use ASM, but Oracle really seems to be pushing for it. We're in the midst of trying to figure this out ourselves, and the physical DBA assigned to making these decisions describes the ease of installing RAC using ASM, and the extreme difficulty otherwise. The Oracle rep suggests that we avoid our SAN altogether and use ASM to stripe and mirror onto 2 separate NAS installations. Phew, but definitely cheaper!
I'm suspicious that Oracle can provide for a high performance solution in this area, as the hardware in our SAN is already optimized for this functionality. Everything we've seen on paper indicates there's really no performance degredation, though.
Like I said, I'm the skeptical one. I see this as Oracle's attempt to "round-out" their product by incorporating the Operating System world. There have been rumours of Oracle reaching it's legacy phase now that MySQL and it's competitors are coming of age. I think that ASM is one way that they're trying to keep their product moving in a new direction. Just my opinion though.