Well, leaked builds can change. 'Allegro' was the codename of Mac OS 8.2 - and Apple decided to rename it 8.5 when it still was in Alpha stages. So it happened before. I voted 10.5, but I'd also understand that they don't want to reach 11 so soon. (They could still go to 11 directly from, say, 10.3, without ever going to 10.5 at all.)
Version numbering is a marketing issue rather than a 'logical' issue. Okay let's *not* go to Microsoft's naming scheme nightmare ever since Windows 95, but let's look at Adobe. ImageReady was a separate product at version 1.0 and was bundled with Photoshop at version 2, I believe. PS6 shipped with IR3, PS7 is shipping with ImageReady 7.0. See?
Now, I still think that Jaguar will be named according to price (not features). If they decide to make this a full upgrade (with your Mac OS 10 or 10.1 discounting the price), it will be 10.5, as a 10.2 version could not be sold for much money (again, not because of the feature set). If it will be 10.2, the price will be lower.