Wow, no one replied to this! I don't really go to concerts, and I don't know what state or province you're in, but at SARS aid a bunch of people got heat stroke. Whenever my friends go to concerts they normally just wear jeans and a t-shirt, I suspect that's what most other people will wear, I would probably just wear shorts and a t-shirt and be really casual. (I think Live 8 is like an outside day concert type of thing but I'm not sure)
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"...If I know my supermodels (and according to the half-dozen or so draped across my bed in a jenga of crack-glazed longing, I certainly do)"
Are you going to the one in Philly? I don't think it's supposed to be too hot on the east coast this weekend so you might be ok with jeans or light pants. I usually try to wear closed-toe shoes at big concerts because there's the danger of having your feet constantly stepped on.
Or if you don't want to wear jeans, Bermuda shorts could be a good option.
For shoes, something comfortable but not too athletic like these could be cute: