i don't think so. i think they're just hugely popular in general, at least around here. i think out of the last 25 or so brand new engagement rings i've seen, all but one was a halo.
not a cluster (like Diana's ring)... but a halo.
and everywhere i go, the rings i see, are all halos. usually princess cuts.
... i actually had a waitress say to me a few months ago that she didn't know diamonds came in any shape except square.