5.1 and apps will have to be certified specifically for those handsets. This means, just like every other app store out there, that Tango users won't see incompatible apps in the Windows Phone Marketplace.Tango is designed for lower-end devices with less RAM, which means that some apps, most notably certain games, won't be able to run on Tango phones. This seems to be enough for Microsoft expert Paul Thurrot to claim that WP is moving towards fragmentation, because Tango phones won't be able to run Plants vs Zombies. Tango is expected to be Windows Phone 7.
If this is the standard for a "fragmented" platform, then every platform on Earth is fragmented, because there is no platform around in which every device in the ecosystem is running the same OS version. And, not having access to high-end games on a low-end rig is a fact of the tech world, not a reason to cry "fragmentation". If you want to play Crysis 3, you had best own a quality computer. The same philosophy goes for mobile devices, and that's not a bad thing.
Either way, we'll know more about Microsoft's plans, because Tango is expected to be unveiled at MWC at the end of the month.