Not strictly true.

For "compatibility", Win98 assigns hard disk drive letters in exactly the same way as DOS. It doesn't do the same for CD-ROM drives. This has nothing to do with the BIOS (unless you're running in 16-bit compatibility mode).

The order's a bit complicated. See this or this for more details.

It's a bit skimpy on what happens if you have multiple primary partitions on a single hard disk, but since DOS fdisk doesn't let you arrange your partitions like this, I guess it's academic for most people.

Edit: found a FAQ at Seagate with more info