I was this summer on an outdoors blues festival, where the main band was ZZ Top. I had contacted the venue beforehand regarding camera policy and when I finally got the answer, it was that (D)SLRs and video cameras were absolute no-nos, but P&S cameras were OK. So I took only a Canon P&S with me. Well, at the site I found out there was no control at the gate and the field (about 12 000 people) was full of DSLRs and video cameras.
I guess the only way of really knowing is contacting the venue AND people who have attended the festival before (if it's a yearly thing).