I think that following the trend of the game releases, with M2 being released Q4 of 2006, and Rome Q4 2004, we can safely assume Empire will be released around Q4 of 2008, or sometime in Q1 of 2009. Needless to say, the aforementioned is deduced from previous experiences, and is by no means certain. I imagine the addition of Naval warfare, and with that the need to significantly change the engine in its current form, will take up a lot of time - which can of course push the release back to somewhere in 2009.