Mine will do this too if it has sat outside on a cool night. A cool night is if the temps drop below 10 degrees. When it gets that cold, it is almost guaranteed that the speedo will stick. From what I have read, there is a delamination issue with the face of the cluster. The face plate pulls away a bit and puts pressure on the needle, not allowing it to move freely.
Just from reading around on the forums, it seems to be most common on the 2006 lariat clusters. If your issue is happening only when it is cold outside, Ford will tell you to replace the cluster, which isn't cheap. I read a post a while back where a guy took apart his cluster and used some sand paper to sand down the backside of the needle pivot point. This created clearance so that the face plate wouldn't interfere anymore. I haven't tried this yet, but plan on it someday when I have an afternoon to kill.
I just have one of my parameters set on my scangauge to MPH all the time anyways. Seems that my cluster always reads 2-3mph faster than my scangauge & gps.