Changing access privileges not working

After trying to change rights on my Documents folder, so that all files in that folder be readable and writable by a second user, my own write-privileges to files inside my Documents folder are hosed.

I applied the settings to all documents inside my Documents folder, and when I go to any file inside it and check the Informations, I, indeed, am supposed to have *read & write* privileges. However, whenever I want to change a document and save (read-access works), I'm being told that I don't have the necessary privileges (in contradiction to what the Information window tells me). Creating new documents does work but I can't change any documents. The same goes when logging into the second account to which I attributed read & write privileges. That second account does have read-access but write-access does not work.

I tried logging out, repairing authorizations, rebooting, sudo chown -R Documents/ , and sudo chmod -R u=xrw Documents
This is on a 10 days old install of Leopard, starting from scratch (copying only Documents and a few manually selected Library documents, i.e. very clean install).
I really don't know what to try anymore, and of course, not being able to making any changes to documents is a serious issue. Any help would be appreciated...