Whatever the cause, it takes over the entire Finder, preventing me from opening or working in any other applications. I have found a way to bypass this annoyance. I use the option-command-esc key sequence to bring up the "Force Quit" window. Then without force quiting any apps, I simply close the force quite window. for some reason this frees up the finder again, and allows me to work on other apps while what ever app that is lagging behind and causing the temporary lock up, catches up.
[Editor's note: I have no idea if this works, but if you get the spinning rainbow in the Finder, it's probably worth a try...]

