MacOS – How to turn ~/Documents into a symlink

I keep my Documents in an encrypted directory in Dropbox (~/Dropbox/encrypted), which I then mount at ~/clear when I log in. On my other Macs, I have change ~/Documents to a symlink that points at ~/clear, but trying to do the same on a new Mountain Lion machine doesn't work:

Pertinax:~$ pwd
/Users/drew
Pertinax:~$ ls
Desktop         Dropbox         Music           README.markdown tmp
Documents       Library         Pictures        bin
Downloads       Movies          Public          clear
Pertinax:~$ ln -sf clear/Documents .
ln: ./Documents: Permission denied
Pertinax:~$ ls -l@hd Documents
drwx------+ 3 drew  staff   102B Apr  5 15:02 Documents
Pertinax:~$ xattr -l Documents

It doesn't appear to have odd permissions or extended attributes, so I'm stumped. I don't recall what I had done previously to make this work on my Lion and prior machines, or if I did anything special at all.