If we have to embrace the idea of ridged Rommies, then I'd go for the idea that it's a genetic mutation that is present in some of the population while others look just like Vulcans. It almost makes sense. But I wish none of them had the ridges at all.
I've sort of gone through different phases about that.....when I first saw the "new" Romulans in TNG, I thought the ridges looked totally stupid. Then after I got used to them, I started kind of liking them (as recently as my first post on this forum a week or two ago, if you happen to see it and wonder why my opinion has changed) though I've always thought they could be toned down a little. Like, thinner, and not as high. In the last few days, I've started thinking of them as kind of stupid again -- and not just for aesthetic reasons. Two thousand years (I think it was established somewhere that Surak was doing his stuff on Vulcan at about the same time as Jesus Christ was doing his stuff on Earth) is not enough time for those mutations to have occurred, and considering the xenophobic tendencies of most Romulans, I don't think they would have interbred with aliens to the extent necessary for the ridges to be so prevalent. And also, I agree with what some other people have said in this thread, that it would have made some interesting plots for them to be able to pass as Vulcans.
Someone said something about accents; I kind of liked Donatra's accent in Nemesis. It wasn't very pronounced, but it had just enough of a difference to it to be a little exotic. And the Romulan language in Diane Duane's (sp?) books was very flowing and fluid, which is the impression I got from Donatra's accent.
Oh, and by the way, keep those Ruffles jokes coming, I'm loving those.