It's already been announced that presales are to be handled by Fanfire.com (though the sign-up to Fanfire might be through U2.com), for which a membership fee will be payable. For the Rolling Stones tour last year (not sure if this was Fanfire, but the principle will be the same), the membership fee was $95, which gave priority booking of 4 tickets to only one show - hence fans wanting to book multiple shows had to pay the fee multiple times. There seems to be an expectation that existing subscribers to Propoganda will get some kind of concession, though we don't yet know how this will work.