Problem
After upgraded to Ubuntu 9.10, I could not update any wordpress plugins any more. It worked fine with early Ubuntu releases. After a quick check on the web, it was a bug in the Ubuntu release. 
Solution:
Ubuntu has not put the code into the final updated yet. You need to add karmic-proposed into your apt source file.
Here are the two lines you need to add to your apt source:
>sudo nano /etc/apt/sources.list
deb http://security.ubuntu.com/ubuntu karmic-proposed main restricted universe multiverse
deb-src http://security.ubuntu.com/ubuntu karmic-proposed main restricted universe multiverse
After saving the file, run the following commands:
- sudo aptitude update
- sudo apt-get update
- sudo apt-get upgrade
After the above steps, wordpress has no problems to update plugins.
Reference