otto - two things... First, the Traverse (and in fact, all 2009 Lambdas) should get slightly better fuel economy because of the change to a Direct Injection engine. In most applications, it appears to be making about 1-2 MPG difference. So instead of 16/24 (FWD) you can most likely expect EPA estimates of 17/25 to 18/26. (AWD is a bit less)
As far as real world numbers, like zman says, it all depends on how you drive and your local terrain. In the DC suburbs, we get an almost constant 17 MPG in 100% city driving, about 19.5 in mixed driving (70% city, 30% highway), and hover around 24-25mpg at highway speeds (60-75mph) in our FWD 2007 Acadia SLT1. It's mostly flat terrain around here and that's using a light foot with easy acceleration. I can VERY easily drop that to 12-13mpg by using jackrabbit starts and giving it more gas than needed to get to to speed (basically, by driving it like I do my G35 coupe).
The bottom line is, the EPA estimates are achievable if you drive it like you intend to conserve fuel. Without thinking about it and just driving normally with 70-75% stop-n-go driving, you can expect about 1mpg better than my avg with an 09 model, so 19-20mpg, again, depending on the weather, terrain, etc. The truth is, there's NO other SUV this size that gets better (or even equal) gas mileage (the Yukon Hybrid comes close, at twice the price).