I, and it appears, many others are getting the same type of error message and even doing the following suggestion locked me out of my WP install altogether:
Question: How do I upgrade automatically from WordPress 2.7 to the newer versions coming out (ie: 2.7.1)?
Answer: Upgrades are built in starting with 2.7 and up. Tools->Upgrade.
Make sure you Create the /wp-content/upgrade directory (it was not present), set permissions to 777, then update the permissions in wp-content/plugins to 777 as well. Finally, I emptied /wp-content/cache - and plugins are now updating successfully.
Well, I tried all of that and every time I did I was unable to get back into WP so I changed the permissions back and things were normal again, although upgrading automatically seems an impossibility at this point.
Why is this such a kludge? I long for an update to Keith's fabulous automatic update plug-in.