We can't use the speeds of parallel ATA because it's parallel. It's alot more effort for the computer to time the signals to have them run down the cable in perfect parallel, vs a serial connection, where they can just stream down one after the other.
Not only that, but Serial ATA will have adapter dongles, that you will be able to put your Parallel ATA drives in Serial chains.
It will be a GREAT thing, once it comes out. I would be all about it coming out now... but I don't have the upgrade cash, so I really don't care
