Im photographed a local music festival last year and doing it again this year.
I had a lowepro trekker II backpack, but it was too fiddly to get into for lenses, batteries, cards etc.
I got a lowepro stealth reporter D200 AW shoulder bag for this time round.
it is plenty to hold my 40D with sigma 24-70 attached, 50mm, 85mm, and 430EX flashgun ,plus memory cards, couple of spare batteries etc.
Although im not sure if the D200 AW would fit the 40D with the grip attached aswell, but i found the camera and grip were out of the bag most of the time, but the D300, D400 etc are bigger versions of the same bag.
But it's small enough to carry round, and not be bumping around all over the place, plus it's got a zip across the top for easy access without having to open the bag.