Our last milestone-related press release (en 500K) received virtually no attention. Most major newspapers and magazines have already run stories on us, it's not like we need to tell them we exist. Why not save the press releases for when we actually have news?
No, the last milestone-related press release was the Commons reaching 100,000 uploads, which, despite not being sent out to many places (http://commons.wikimedia.org/wiki/Commons:Press_releases/100K/log-en), did see some press coverage, mostly in Germany (http://news.google.com/news?hl=de&q=wikimedia)
Also, as Przykuta notes, the milestone will be shortly before Wikimania, so the release need not focus only on the milestone, but include details of the conference, since we should know by then who most of the speakers are, meaning we will have something other than a new milestone to announce.
Angela.