I have been having this same problem since at least 10-5-07, when I first filed a bug report. I have reported this bug twice, and the Sketchup team was not helpful either time. The first time, they told me to update to the latest version, even though I was already on the latest version. The second time, they told me it this problem occurs after Sketchup crashed. Which has not been my experience.
I am on Leopard and a Macbook Pro.
edit: I am using only Wifi, and am using the latest version of Sketchup.