Windows Phone "Tango" aka CR3 is the OS update everyone wants though very few know why. Indeed it brings only a handful of updates, mostly revolving around MMS options and geolocation icon. The OS update is mostly aimed at the low-end 256MB devices that are now hitting the market though it does give companies a chance to tie in precious firmware updates with the OS.
Having said all of that the Finish Tango ROM (8773 with firmware 12140) popped up on Navifirm this morning and yes, can be downloaded. Does that mean it can be flashed to your AT&T Lumia 900? We wouldn't recommend it as down-grading the ROM to the proper one is not guaranteed. Still, we know many of you are anxious on this end so we'll see what the more ambitious folks have to say if someone dares to flash.