There's an interesting article up on the Alive & Knitting blog by an apocalyptic SFR author Claire Delacroix about Science Fiction Romance vs. Space Opera Romance. I thought her definitions were interesting and certainly had merit. It seems some of the SF crowd (I could be wrong about that) immediately jumped on the bandwagon with their battlecries of "you're wrong!"
Nope, I don't think anyone can clearly delineate the various subgenres so--of course--I had to offer up my slant, with a good dose of MHO. :)