Sorry to get all scientific, but some of the posters here are confusing weight with mass.
The extra fuel at the start will make almost no difference to the downwards pressure on the tyres, because that is mainly provided by the aero.
It will however greatly increase the mass of the car, which brings greater inertia. The tyres will have to do more work to accelerate the car, slow it down and change direction. That is why there will be greater tyre wear, but slower lap times.
It's very hard to predict what the best strategy will be. It partly depends on how close the field is, and what strategies other teams might run.
Imagine a car which is running at the head of the field. They want to make a tyre stop, but if they do, they will immediately lose 10 positions, because the field is running close together. They will have more pace with the new tyres, but they cannot use it unless they can do a lot of overtaking (hardly possible with current cars).
That will not be a disaster if those 10 cars then also pit within a few laps. But if they don't, our hero is stuck in the midfield.
An alternative would be to keep running until all the other leading cars have stopped, then quite possibly pit without losing the lead.
My guess is that most will want to wait to see what is happening. If a chief rival has pitted and is flying with no traffic, you wouldn't want to delay.
Of course, safety cars will mess up this strategy. If you're the only guy who hasn't pitted, you will be in the lead but on old tyres, and with the pack right behind you.
Actually, scrub that last sentence. If 2009 safety car rules apply, you can pit immediately, so no problem.
Edited by BillBald, 01 January 2010 - 14:33.