I dont think it will work quite like that. There will be an amount of Powerbooks/macs in general that will have been sitting in a warehouse for a few days - weeks that will still only come with 10.3. Not to mention any macs in transit. These things take a good week or two to get from the production and packaging lines to your door step, via various distribution points and hubs throughout the world.
So I would expect it to be something like this. Tiger is announced, if you order a mac in next week or so it will not come with Tiger at all, pre-installed or on a disc. You would be eligible to get Tiger for the cost of P&P only. If you wait a week or two, any mac you order should come with Tiger pre-installed.
We really could do with a sticky about this, based on what's happened with previous releases.