I have raised a ticket with trac for this, the answer so far has been that it is a server issue, but some plugins update and some don't.
What has happened in my case is that by using ftp after the failure I can still see a folder that could not be deleted, this cannot be renamed via ftp or written to.
In about 20 - 30 minutes this folder vanishes and I can add the plugin back, this also happened with the twenty eleven theme upgrade, so I am sure it is the WordPress update function that is causing this file locking.
I also have a topic open for this in the forum.
The forum responses tend to ignore what you write and leap to the defence of WordPress, the upgrade process has always worked for me, until the WordPress 3.2 version release.
My ISP has said that nothing they are aware of has changed on the server to lock the folders or files and the updates should work ok.