I attended an alt.net usergroup the other day. There was a lot of dialog of what alt.net is and if the alt.net movement has already started dying.
It doesn't really matter if there is a movement. Alt.net is more about concept and a way of thinking than it is about a bunch of people. As long as people develop tools and components to use with .net and don't just wait for the next release from Microsoft, the spirit of alt.net will live.
So if you are a .net developer and your primary source of ideas comes from Microsoft then you don't have to care about what alt.net is. If you on the other hand keep looking out for better ways to do stuff, an alt.net meeting might be for you.