Well, I suppose I could make do with just 500mm but it's got to be at least an f/4. This is what I'm hoping Pentax has in mind for its "super telephoto" lens. Still, that 600mm f/4 would be a cracker to shoot with even if you do have to hire a Sherpa to carry it.
If I had one I'd give it to you.... if you came over to pick it up.
I'm trying to get rid of whatever I can't lift, and that lens would be one of those things