This year, Bonnaroo started serving cocktails, and I couldn't have been happier. (In previous years, they only offered wine and beer.) Almost every beer stand also sells whiskey lemonade and a refreshing vodka-tea concoction, and specialty stands offer even more variety.
If you're camping, you can bring as much alcohol you want into the campground, as long as it's not in glass containers and you don't bring it into the actual music-festival area. No one bats an eye if you want to mix up a vodka soda for the walk to the gates, as long as you finish it before you enter.