Just discovered this myself a few days ago and I wish I new about it beforehand so I could use it this year.
From what I can see is that if you're in receipt of middle/higher rate DLA and you buy a festival ticket you can then apply for a free ticket for a carer so that you can enjoy the experience and in some peoples cases even go due to the help of this person.
I think it's a bloody fantastic idea!!
So far I can see that Glastonbury, V fests, Carling fests, T in the Park, Download. I'm sure there is plenty more out there and it wouldn't surprise me if all of the festivals are now implementing the system.
I believe each festival has a different way of running the scheme so be sure to check their websites.