At a couple of technology expos like Intels people have noted when you move the cable to the serial ata drive the computer reboots. Serial ATA may still have a few kinks to work out. I have not seen any information on vendors selling Serial ATA Hard Drives yet????? Why should you volunteer to be a Serial ATA Beta Tester? The only advantage is really thin cable.
Inside a hard drive there is a UART device that converts parallel to serial already, so I dont think that will slow it down much. Howerver you would be going from paralless to serial at one end then from serial to parallel at the other end and from parallel back to serial inside the harddrive. What a Hassle. Still 1.5 Mbps it is probably still faster than ATA133 even with the adapters. I saw somewhere where one of the RAID card manufacturers had tested it already on normal hard dirves.