I have been fighting this problem several times, without success.
Now I have been able to pin down the problem.

The wanted behaviour is:

    You paste the url of the module or theme you want to install
    You click install
    Your module or theme gets installed

What I have seen on several installations, and this is what you don't want:

    You paste the url of the module or theme you want to install
    You click install
    It asks for some FTP (or SSH) credentials. WTF??????

To avoid this, make sure the folder /sites/default is OWNED by the user that executes the drupal scripts. On most Ubuntu installations, this is the apache user: www-data.

Don't ask me why this works. I don't know if this is intended or not. It doesn't make so much sense to me, but there might be an explanation. For me it would be OK that the temporary directory is writable by the apache user and the themes or modules folder is writable as well.

So this should do the trick when the user is www-data:
chown www-data sites/default

I hope this works. Please let me know.