It's obviously 30 million monthly pageviews (I said 1 million daily pageviews, I think you misread my text).

100 servers would assume media hosting for images and other files that go with a blog. That could all be kicked out to other services like Amazon, but the costs would obviously go up and I don't think it would be necessary.

At $500k +/- a year in infrastructure costs, I think Posterous could be wildly profitable. You could run a tight ship with five to ten people (throw in $500k to $1m in employee costs). If someone put a gun to your head, it could be maintained with a two or three person team.