I'm pretty sure they do not. I can remember at PPIV that was an issue especially when the ATM ran out of money. I don't remember any vendors taking credit last year, but I found a lot more ATM's outside the venue.
what AMBR said -- there are ATMs. However, I know for sure that The Laser's Edge takes credit cards (for any purchase amount, I believe), and so does Viagra Records (if they're there; I can't remember).
Laser's Edge has in the past, and I imagine they will again this year. Though Ken should be able to take the rest of the year off based on the amount of CDs Mare and I bought at Nearfest...
laugh if you want... they have quite a nice selection. At BW&BK, I bought Hitt Man - s/t from them (which I had been looking for for years), and I found Superior - Ultima Ratio for $10, sealed. And that's not to mention all the cool stuff they had, that I couldn't afford.
It's actually Vyagra Music (with a Y)...and they FUCKING RULE!!! I'm heading up to Philadelphia in 23 days to see Edguy and you better believe I'll be visiting that damn store!!!! Hell yeah!!!!
In 2000 I went to the Metal fest in san bernardino and there were Century Media, Napalm, The End, Necropolis and many more, and I found a lot titles at great prices, I hope that this happens again.
I'm pretty sure that merchants are not allowed to implement a "minimum purchase" policy with major credit cards; it's part of the deal the merchants signed when they agreed to accept credit cards. Though I doubt that reminding a merchant of this will have them change their policy.
The reality though is that they don't have the necessary equipment to easily deal with credit card purchases. When I bought a bunch of stuff from them, Heather had to call the cc company on her cell phone to get the authorization code. I can see why they would not want to do that for every $13 purchase.