Oh, yeah, forgot to mention that: One of the attendings at Bernie Ackerman's place (dermpath) is a DO (but can't really remember there's been anybody else but her in the program?). So it's certainly do-able, but there's just not that many around. As another poster mentioned, it's perhaps a combination of the primary gearing of DO's towards primary medicine, and partly a cultural thing.
I know there's incessant discussions about MD vs. DO on SDN, which is somewhat amusing, since every legal authority in the land, up to and including the Supreme Court, has ruled that DO have same rights as MD's.
Personally, I don't know much about DO training, but can't see why they shouldn't be able to become excellent pathologists.