I read somewhere in the annals of nerddom...I forget where, but the source seemed fairly reliable...that it's Patty, and the plot of the episode involves her falling in love with a female golf pro, and the whole gay marriage issue will be central. Seems likelier than any of the other options, as officially outing Smithers at this point would be totally redundant, and leave him with nothing to do for the rest of the series.
They've dropped a couple of hints that Patty is a lesbian (and not just the butchier of the twins...) ...in one of the older Halloween shows she said "there goes the last lingering threat of my heterosexuality" when Homer ran naked through the kitchen - and in a later one, both she and Smithers were on some kind of "in the closet" float in the pride parade.
She did go out with Principal Skinner briefly, but told him she wouldn't leave Selma "for any man". Hmm.
Who knows, whoever it is, maybe the show will handle it well, as more than a cheap gag. Or maybe not.