In my opinion, looking at past releases for Apple, there are too few features listed for a 5.1 release, the patch notes look more like a 5.0.2.
That suggests there are features hidden in the back ground, major somethings, like how Apple hid Siri in 5.0 from the public.
I'm thinking we see another beta and a GM with tweaks, and then 5.1 launches in March adding support for a new iDevice.
Now that we are nearing the end of Jan, if they release 5.1 now, it will be a bit unusual short gap until 5.2 to add new iDevice support.