From what I've heard, there will be adapters to make Serial ATA drives backward compatible with standard IDE cables, and adpaters to make serial ATA motherboards work with standard hard drives.
I would guess that we'll see Serial ATA drives in Dell and Gateway computers first, before they show up in the general market. Size will be the same as current IDE drives at the time, and price will most likely be a little more expensive at first. That's my guess.