An anonymous user posted the following as a suggested edit to one of the answers to this question:
This seems to be related to Avatron's Air Display, see this post.
From the linked web page:
Here’s the current status: IF you’re running Lion AND if you have an NVIDIA 9400M/9600M chipset AND you have a third-party video driver installed, THEN you will not be able to release the mouse button while drag-dropping something.
Well, it turns out that the drag-drop bug only manifests itself if you haven’t done a Wake From Sleep since your last Log In or Restart. So there’s a workaround: After each Log In or Restart, close your MacBook Pro, wait for it to sleep, and then open it again to wake it. It’s a little bit of a pain, but it’s there as a workaround if you want to use Air Display on one of these Macs.