There is probably no official page for that, but the android market
description tells you everything you need to know.
Osmand (without +) is in version 0.5.3 and will not be updated. Every
feature which came after is not in it.
Osmand plus is in version 0.6.8 avaible and you get allways the newest
official release as update.
You can also get the newest versions from the official website:
osmand.net or if you like to test and contribute there are many
nightly versions in googlecode.
There is also question about GPL. Answer is yes, both are under Gpl
v3. You can get the source from the googlecode page. Just follow the
link on
osmand.net.
I will answer the question on osm for everybody.