so here's the official rule:
here's the video: it looks to me like he got both feet down. There's a lot of debate, and most folk seem to be arguing that it was essentially a "tie", his foot touched the ground at just about the same time the ball came loose (http://deadspin.com/5878309/did-lee-evans-catch-a-game+winning-touchdown-for-the-ravens)
I'm okay with the call. Evans should have held onto that one. It was a perfect pass by Flacco. The only problem that I have is that it was not reviewed. Kind of surprising that a play that close doesn't merit review when just about everything else does in the final two minutes.