I've got a feeling they might have some new aero parts, we also don't know what sort of fuel loads they're playing with yet.
IMO it definitely way too early to predict how the season will pan out. Teams like Ferrari and McLaren have incredible resources and data at their disposal and will undoubtedly find a few precious tenths before the circus even gets to Europe. Of course other teams are also going to find a few extra tenths but.... how quickly. The reason I say how quickly is because Brawn/Williams/Toyota are already using the rules interpretations to their advantage as deemed legal by the ICA.
If I were part of any one of the other 9 teams, I'd be worried most about RBR. Mark Webber has potentially never been in a better seat, especially if he gets himself a contract for 2010 (with RBR) if he hasn't done so already! I just have a feeling that RBR are going to establish themselves as a front runner.