Solution:
I just found these: link 1 and link 2, where I dug out this:
WordPress asks for your FTP credentials when it can’t access the files directly. This is usually caused by PHP running as the apache user (mod_php or CGI) rather than the user that owns your WordPress files (suPHP or FastCGI). Switching over to using suPHP or FastCGI would probably fix it. Of course, if you don’t want to mess with that, you can always just put your FTP credentials directly in wp-config.php or use this in the shell:
pae$ cd /var/ww
pae$ sudo chown -R www-data:www-data wordpress