This is just a theory I have had for a while now ever since the lineup was delayed and I thought I'd just share it with you. It's not that I am dying to see U2 or am really that big of a fan but I do think that having them on top of the lineup over The Killers would make much more sense.
Ok, so my thoughts are that Goldenvoice was trying to work out a deal with U2 and Live Nation to get them to headline. I think (and this is all speculation) but I think that U2 wants to start playing festivals. They have toured the world over, they have sold out stadiums all over, but they have nott done a major US festival and I think that they want to. There is no reason for them not to and they are one of the only Major bands (Radiohead, Pearl Jam, RATM, etc) not to play one yet. Now being in the position that they are in with Live Nation kinda restricts their freedom to do whatever they want but I am pretty sure there is some sort of "escape clause" for certain situations. I could see Goldenvoice trying to negotiate a deal with Live Nation and I could actually see Live Nation being difficult to work with in this scenerio. (there have been rumors going around for years that said that Live Nation was trying to buy out Coachella from Goldenvoice or Goldenvoice altogether)
U2 would probably play for 1.5 - 3 million which I would imagine is in the budget for Goldenvoice but I think that Live Nation wants more out of the deal (sponsorship, future deals, etc). This, in my opinion, is why the lineup was delayed. I can't really see any of the other artists on the poster having a difficult time negotiating out a deal.
Since U2 hasn't released any tour information this possibility could still be on the table. And I think that if they haven't already, Coachella could very well have struck a deal with them and are just waiting for the right time (release date of the album) to announce them as the Saturday headliner. I dunno. It could be cool seeing U2 out in the desert 30 miles from where the inspiration to The Joshua Tree was made.
Like I said this is all speculation so take it with a grain of salt.