A couple notes--sorry I'm late to this discussion:
I don't think a band who would lop off Mercy and Fast Cars to get an album well under an hour are really the kind of people who would consider a double album.
Having said that, they were in a more "pop" mode back then, and wanted to distill things down to a finer point. Maybe they're feeling more ambitious now, but I'm still having a hard time seeing it.
Also, I don't know if bringing The White Album into the conversation is a good comparison. While I think it's one of the better double sets (and I agree with the person who said that often on doubles the "filler" adds to the experience), The Beatles were in a completely different recording mindset. George, Paul and John were all off writing mostly alone, often bringing nearly finished songs for the other to complete. And the sense of oneupsmanship probably led to people fighting to keep including their own constructions. When a band is writing everything together, like U2, it's probably easier to discard things that don't fit a general tone they're trying to achieve. In the case of something like The Wall, Tommy or Quadrophenia, there's obviously a concept that requires a certain amount of pieces to tell the story.
Lastly, someone said earlier in the thread that all the songs are going to come from post-2006. That's totally not true, because in the interview Edge said that half of the material was coming from stuff they'd been kicking around for a while, things that have been rewritten, rearranged, etc. And while that does give hope to the Mercy crowd (myself included) we should be careful what we wish for because it will probably be quite mutated from what we know and love.