One immediate problem with that idea is that you need someone whose motherboard is RAID capable, else you need some sort of logic inside the HDD to work the RAID...
Computer manufacturers have a habit of going for the lowest common denominator, which is the basic motherboard with no on board anything... just the basic essentials in the slowest forms. HDD manufacturers are making drives that will still work with ATA33 for Christ's Sakes!!!
Another problem is that if the HDD logic was implemented, it would lower the Mean Time Before Failure (MTBF), which is the standard that all drives are measured against, and the reason why IBM drives are getting a hammering recently.
Oh yeah, and finally, it wouldn't work
RAID 0 relies on the two disk heads being able to read and write both two bits at a time in order to speed up the data handling capabilities of the system... In order for RAID 0 to work, two disk heads would be needed, which would bump the cost up even more!
RAID 1 is totally not possible on a single drive, because it is a data protection system. RAID 1 duplicates the data across two drives, so if one fails, the other still has the data to recover. If you use a single drive and it fails, you're knackered...
Because RAID 1 would be impossible, so would RAID 0+1, which does RAID 0 on two drives, and copies these drives onto two separate disks...
Sorry about the length and disorder of the post, but hey, you get what you pay for