My understanding is that a SS with CB only is a bomber.
A singlespeed with front&rear brakes is a klunker
A ballooner cruiser that has been modded to the point that it has front and rear brakes, plus derailer gears is a hybrid.
Of course, I think this is largely revisionist classification, brought about by modern folks who are accustomed to a lot of sub-genres existing within each genre of biking taking an interest in these primitive trailbikes. When I watched the Klunkerz movie, it seemed to me thatth ese terms were kind of informal and basically non-standardized; different folks used different terms for the different bikes.
In the end, I suspect that a lot of ppl contributing to this thread are building bikes to an aesthetic, rather than actually doing their best to make a trail-worthy machine out of a re-purposed cruiser. Folks rocked bombers b/c they were broke or b/c, early on in the mountainbiking craze, they didn't know what worked just yet. If you've ever spent about 5 minutes riding offroad, you know you want more than just a coaster. The derailer gears thing is something that can go either way, but when I see ppl with a 3speed "klunker", I know they're going to kill the hub if they ever ride it for real offroad. Those hubs aren't designed to shift under load, they aren't designed for >2:1 gear ratios, and they aren't built for the kind of abuse that trail-riding dishes out. Show me a 3speed klunker, and I'll show you either a brand-new build, or a bike that was built for show rather than go. (Nothing wrong with that; those bikes tend to look pretty cool...much better than derailer-equipped versions.)
I think it's weird that the original klunker-type guys were doing their best to advance a new sport; they were using old-frames, but a high-tech approach. In trying to replicate their bikes, we are actually doing the opposite. If those guys were born 35years later, they'd be on cutting-edge, modern mountainbikes today. They rode Schwinn Excelsior frames because actual mountain bikes hadn't been built/invented yet.