New Project Brings Holo UI to Instagram, Snapchat and more (available for download now)
A couple of guys on XDA Developers have successfully made modified versions of Instagram, Snapchat, Steam and Grooveshark that comply with the Holo UI theme on Android.
The community appears to be growing rather rapidly, so it's definitely worth joining their Google+ page and recommending what apps they should do next.
I wrote up a quick article about it here
(sorry if this has been posted already")