I started running a blog at http://www.jeffschult.com/blog in December. Among the plugins I've been using is wordpress.com stats -- the one that uses wordpress.com's processing with the API key. It's worked fine on that site since Day 1, never a problem.
BUT -- I have since launched two other blogs.
Both are using the stats program. Both have my user API key. Both installed and activated without incident.
I have the choice of viewing stats for all three sites when I click on "Blog Stats" on my Dashboard. However, the two newer sites have never shown any page views, hits, clicks, etc. though I know they have received many. They look fine. They just show no traffic.