Like others have said, I too can see bulkhead seats on RJs. I can also see just about any exit row seat in coach. I have always been able to select the bulkheads in FC when I get an upgrade or purchase a FC ticket.
It's the coach bulkheads on full size airplanes (MD88s, 737s, 767s) that I've never seen available.
I'm intrigued by Berto's comment, that he can book bulkheads on delta.com but they warn that he could be bumped.
As for the reason why FC bulkheads are available, but coach one's generally aren't, I think this has to do with how the ADA works. Airlines are required to accommodate those with disabilities, but they probably are not required to provide premium cabin seats.