The seed will germinate, because I have them come up all over the place in flowerbeds near other plants.
By far the surest way to propagate this plant (and its cousin the foxtail) is by division. Take a full plant and cut it into several parts with a SAW. The roots are extremely fibrous and hard to cut through. Plant up each section and it will fill out quickly.
Ahh - I thought it was a coincidence. I got an email last night from another forum participant who saw a picture of one of my container plantings that had A. plumosa in it, asking for a start of that plant. I would have said the same thing DP72 said - easiest is divisions or new stems with roots attached, but they do come easily from the seeds you remove from the berries, too.