You're totally correct Amy. Even if it's not an airline interview you should always dress professionally. I view it as a sign of respect towards the company I'm applying to.
I went to a Virgin Blue interview last year and was dressed professionally in a suit and like most gals out there trying to look my best and not get a ladder in my stocking.
What was a surprise was that some women in the group interview were wearing denim jeans and a girl in my activity group had a denim jacket on. I never heard back from Virgin and maybe they did, but I was surprised by how casual some people were dressed for an interview.
Who knows... maybe that more relaxed approach was the key for them and they're up there now flying around.
Cheers
vdd