Interesting post on AreoXP about Office 14 - which states that we can expect Beta1 to reach us in 2008. Also see this quote from TechNet Radio with Eric Vigesaa:
Michael: So now that Office 2007 has RTMd you are taking a vacation?
Eric: I actually am taking a vacation for two weeks and then, it may sound strange but the next version of Office is already in the planning stages, we want to make sure that our IT perspective and apps are represented in the new product focus and offerings so we’re going to be very involved with that.
Michael: So that’s Office 13 in development huh?
Eric: Well, 13 is unlucky so we’re calling it Office 14.