(Zul'Gurub will be an open, level 30ish area (not instance) when Cataclysm comes out)
"Will the Zul'Gurub mounts still be available by other means?"
We don't know yet. However, if you wanted them and haven't gotten around to farming one (or both), I would suggest you start now, just to be safe. If I had to guess, I'd say they're going to be removed -- but that's just a guess. I don't know either way.
I'll also take a minute to say how scared I am about what's going to happen to the price of the turtle polymorph tome ... I could see that going well over 100,000g eventually. If you have one and haven't used it, now would be the time to save it and forget about it until it becomes a super rare item of yet-unknown demand.