The split between Windows Phone 7.0 and 7.5 has caused confusion, but developers are quickly getting up to date.
Now that both Russia and Japan are seeing different Windows Phone 7.5 models unroll in their markets, Microsoft wants developers to now focus on brining in apps developed for version 7.5 Mango as well as update the apps that were for version 7.0 so they will be up to date with this new system. The problem originally came with the division between the two versions where users on 7.0 would not be targeted by the newer apps and they would have to then bring in the Windows Phone Mango update to keep their apps fresh, essentially meaning that there would be two versions of the app at the Windows Phone Marketplace.
The goal for Microsoft is to begin having developers submit their Windows Phone 7.5 app versions now so that they will available in October, and at this point they will allow for updates to both versions of the apps. Hopefully the Windows Phone 7.5 update to Mango will be available at the end of September, so if developers begin submitting their 7.5 app versions now they will begin to be available when users first try out the update. They are also asking developers to indicate what features are available in the 7.5 version, which may also play into helping usher users over to the new system more smoothly.
Since the Windows Phone 7.5 offers new features like live tiles and the ability to switch back and forth between apps quickly it may really be advantageous to developers to have these new tools. On the other hand they want to avoid the fragmentation that has caused so many problems for the Android systems. The split was intended to force developers to switch over to Mango, and now it seems that they are asking nicely.