This was in the initial post - I think this explains it pretty clearly, but I could be missing something in this discussion.

Beginning in 2010, rather than six systemwide upgrades, Platinums will now have the option of receiving four new systemwide upgrades or can choose from a set of optional benefits instead. For those current Platinum Medallions who earn the new Diamond Medallion status, in addition to the four systemwide upgrades, they will receive two additional choice-benefits including the ability to have six additional systemwide upgrades for a total of 10 systemwide upgrades.

So if you qualify for Diamond in 2010 and you select SWUs for Platinum and Diamond, you will get 10 total for 2010.