Tickets are free.
You can enter to win one of 25 pairs of tickets here; drawing will happen 7 days before the show:
http://www.itunesfestival.co.uk/lineup.php
Hundreds of tickets will be given away on Facebook and through an iTunes app:
http://www.facebook.com/iTunesUK
http://www.itunes.com/FestivalApp
For those of us poor souls not in the UK, "Recordings from each gig will be available to download on the iTunes Store."
Also, "The gig will be broadcast live online via the
iTunes Festival app and on ITV.com"