It depends on what you are photographing and what you are doing with it, Tom.
(A little background...since you've been absent

I've gotten pretty hot and heavy into the art fair scene - am even considering Des Moines next year.)
If you are taking general photos, like the ones in
this blog (haha, second photo, you can see someone shopping at my booth!) you are perfectly fine.
If you are taking photos of a specific booth, like the one in my attachment, you'd better talk to the person running the booth. This is my booth, but I asked the folks who were shopping if it was okay if I took the photo and potentially posted it on th web. I think the rule of thumb is that if they are potentially identifiable, you are better off asking.
If you are taking photos of "human interest" (i.e., your guy asleep in the chair) and you plan on doing anything with it aside from showing it to your grandkids and friends or putting in a scrapbook, you
should ask permission, but it's not essential.
If you are taking photos of specific art work....you'd DAMN well better talk to the artist. Most artists (myself included) frown very heavily on anyone taking closeups of their work. Copyright infringement, other artists stealing ideas/techniques, etc., are a big deal in the industry. I would be sincerely p-o'd if I caught anyone outside of official fair personnel or someone who identified themselves as a member of the media taking photos of my work. And even those folks had better ask first.
Hope that helps...I'm going to the Omaha Summer Arts Festival this weekend (what's left of it anyway

we had another horrific storm last night!) as a looker, someday I'll make it out to Des Moines!!!