If you are using a kernel >= 2.6.20, all /dev/hd*s will appear as /dev/sd*s. This is a result of Linux using the libATA drivers for PATA as well as SATA from now on.
Simplicity as far as Ubuntu concerned: "This will eventually result in us having to only cope with one "storage" subsystem and make our lives somewhat simpler."
In short, "The reason is that the PATA code in the kernel is extremely crufty. The libata code is much, much cleaner -- and not only internally: libata handles errors a lot better than the current IDE subsystem does, for instance."