This was asked about a couple of times in the press conference yesterday. All the scientists there, both the vis and IR people, were adamant that the only clouds present are those bright features in the south, and that the wispy, fuzzy patterns all over the planet really are surface features. One key line of evidence was the IR spectra -- the clouds are bright and have a flat spectrum, as you'd expect, vs the rest of the moon which is very dark varied in composition. According to them, we're seeing through the high-altitude haze deck, and the skies are pretty clear underneath -- clearer than they expected. They certainly grant that the surface fuzziness is weird and they don't have an explanation for it.

Also, that bright southern feature changes and moves over time (there's a sequence of exposures over a couple of hours posted), which would tend to argue for clouds vs mountains.

They did raise the possibility that the clouds are orographic, though, since they are such an isolated and long-lived patch (previous Cassini and HST images suggest clouds in the same general location).