There are currently two seperate P-51 products.
The first version -- the P-51D and P-51H package for FS2004 -- can be copied over to FSX from your FS2004 directory and can be made to work in FSX. Unfortunately, there are no further FSX updates for this particular package.
A few years later the company released an updated P-51D model under the Wings of Power II banner. This is the version that is offered on the "WWII Fighters: Special Edition" DVD, as a SOLO download product from the A2A store, and on both of the Aircraft Power Pack DVDs. This version can be installed directly into FSX. After installing, download and install the FSX update patch found here:P-51 For FSX forum thread link
Eventually (hopefully by mid 2012 but there is no ETA yet) there will be a completely new Wings of Power III model that will be released in the AccuSim line of products. That product will be for FSX only, and will likely have a small discount for current P-51D owners -- the original Wings of Power version may not technically qualify for such a discount but since I doubt any new release would require an old model to be installed to work, I think the 'spirit' of the promotional offer might allow you to claim the discount if you own either previous version of the Wings of Power Mustang.
_________________I like Thunderbolts!
(Link to MySpace tribute page)120th Fighter Wing, Montana Air National Guard