I admit, waiting 3-4 years between new studio albums is a long time. But it's what U2's been doing for years.
In 1990/91, I had 2 friends comment about the length of time between U2's albums. One person was surprised when JT came out in 1987. He hadn't heard anything from U2 since 1984 (and he dismissed WAIA - and rightfully so, IMO, as it has 2 live songs and 2 songs previously released) and thought they had disbanded. Again, in early '91, friends commented that it was now nearing 4 years since U2 last released an album. I pointed out that R&H came out in late '88, but they dismissed that as it was, in their minds, a "live" album. While I emphasized the 9 new songs, they still considered it a live album.
So for most people, the average wait time between brand new studio only albums from U2 since 1984 has been around 3 years.
But fortunately, U2 fill up that space with other releases, such as WAIA, R&H, OS1, Best Of releases, DVD's, etc.
I feel this is a good marketing strategy. Whenever U2 releases an album, it is big news. Then they are on tour for a year, so they are in the news for a long time. Add in Bono's and Edge's additional charity work, on top of the charity work the entire band and Ali does, and it seems like there is a daily article about U2 or something related to U2. This over-saturation isn't good for anyone.
By taking time between albums, U2 clearly feels that they produce better work. Most artists can't work this way, but I think U2's success (in terms of sales, awards, sold out shows, etc.) speaks for itself. Plus, by releasing albums less frequently, they give the world a little "break" from U2. However, U2 are smart enough to never "disappear" completely. They'll have these little releases (a song, a Best Of album, a DVD, etc.) that keeps them in the public eye just a little, just enough so that the public doesn't get sick of U2, but instead, remains hungry for more U2 material. Brilliant, really.
So as much as I'd love an '07 release, I'm realizing '08 is more likely. And that's O.K. - it gives me time to save $$ for new U2 stuff (buying all the releases and seeing shows is expensive!).