I haven't heard of IDE hitting those speeds yet. Rest assured that if they do come out, heat will be a big issue to deal with on them. Price at first would be high, but they'd come down eventually just as 7200RPM and previous speeds did.
I like SCSI, and I like IDE. I have worked with and used them both. I compare SCSI and IDE like CPU cores. Some just get to a certain point and they can't really go much farther. Like the Palomino core in the XP's. Now they have the Thoroughbreds coming out. I look at it like that. IDE is good, and its fast...but SCSI can just accomplish more.