How can I rename a folder in Lion's Launchpad?
Ok, I did a google search and came across a page where others are having the same problem.

The reply by etresoft - https://discussions.apple.com/message/15661045#15661045 - basically says to kill the Dock application and it should restart automatically, and then it should work.

I did that, started Activity Monitor, found Dock, and force-quit it, it started right up again, and afterwards renaming worked as expected.