2. Demand to the first leg of this tour is phenominally high because they are only playing in 13 cities and missing large swaths of the country. I know a lot of people going to Chicago for the shows that don't live anywhere near there (myself included). This is behavior that would not be occuring as much if they were playing more cities. This demand is causing things to sell out quickly.
3. Work ticket connections in the U2 internet communities before you resort to scalping/ticket brokers-you'll be surprised what you might find.
4. They will be coming back this fall and probably doing more dates in NYC, Boston, Philly, Chicago, etc. in addition to many other cities.
Just some pointers. I've gone to many shows where I've paid no more than face value for tickets and didn't originally get the tickets on the original on-sale date by using a combination of the techniques above.
Don't resort to broker/scalper prices right now either, they are going to be super high right now because demand will be so much higher due to people getting hosed out of the public onsale. Wait a while if you do choose to go this route (and I strongly suggest using some of these other techniques first).