Someone should note in the FAQ that if you experience this problem intermittently, try logging out and logging back in which replaces the cookie.
I normally hit the little check box which says "Save Information" on the login screen, and every once in a while, the whole thing seems to go wacky. I checked the MySQL database, everything seemed to save ok, but entries didn't appear in the blog, etc.
Logging out, then logging back in righted the problem.