AFAIK, it's the 580EX II, to be specific. None of the others in the current Canon lineup appear to have that. Nikon, Pentax, and Sony don't appear to have an option, though Nikon offers a water guard for their flashes. That seems like more of a compromise than anything. On the 3rd party front, Sigma is out (I have one and know) and I can't find any evidence that Metz has a weather sealed option and I know the current top of the line doesn't, so that also indicates.
Beyond that, I don't know, but that covers a lot of ground. Net effect, I think that if you want to seal it, it's the old plastic bag for the vast majority of them...