I got
this one as the first hit. It looks to have a pretty active forum.
and
this one was number two, but looks less active.
There's also sci.space.science and alt.astronomy on groups.google.com. usenet is something of a different animal though. There are unmoderated groups, which are like jungles, and there are pre-moderated groups, where your post doesn't even show up until a mod aproves it, which can take a day or more. Not quite as nice as a web forum.
The
orbiter spaceflight simulator has a forum with an astronomy and science section.
And
apollo hoax is another good place to hang out.