So between Chris Webb's (Kilts subset of skirt) and An t-Ileah's (kilt descended from blankets) theories we can come up with a third theory: that kilts are the illegitimate offspring of a 13th century wanton skirt and a wool plaid blanket looking to sow a few wild oats

Seriously though what we think of as traditional kilt (the infamous 8 yard tartan) is a fairly recent incarnation of the kilt. Check out Matt Newsome's site www.albanach.org for a lot of interesting facts about the development of the kilt. I think it's pretty wonderful that the kilt isn't being regulated to being just a period costume piece. It exists and continues to evolve. Creating new subsets of it's own.

By the by, I think it's fine for someone to refer to a kilt as a skirt in jest...
AS LONG AS THEY THEMSELVES ARE WEARING A KILT!

Cheers