In an interview with 1UP, Paul Wedgewood of Splash Damage told more about the release date of ET:QW: 

1UP: What are we looking at for a release date?

PW: It has actually been pushed back now, so it's not going to be a 2006 release. It's a "when it's done" game. But basically there was a decision to invest more time and money in the game to make sure that it's absolutely perfect. To make sure that the balance, you know, of the maps... they've really turned out to be a bit more of a hurdle than we anticipated because they're so involving and because there's a fair amount of complexity to them. But we want to make sure that they're all perfectly tuned. Now each map in terms of complexity is about the size of four Wolfenstein: Enemy Territory maps, so it's a huge task getting the balance right, making sure that the geometry that surrounds objectives and stuff is working really nicely, and I think it's worth us taking the time to get that absolutely perfect so that all twelve maps are really good fun and, you know, there aren't just two or three or five that people play more than anything else.