Found a working solution here:
Namely:
1. As you've probably already done, uncheck:
"lock screen when screen saver is activated"
in the System->Preferences->Screen Saver menu.
2. Type gconf-editor in a terminal. Under apps/gnome-power-manager/locks check:
"use_screensaver_settings".
3. If still asked for password, you can (also in gconf-editor) go to desktop/gnome/lockdown and check:
"disable_lock_screen"
Credits to itslofty below for this tip!
I only had to do steps 1 and 2 on my Ubuntu 11.04 Natty Narwal on the Acer Aspire 522 BZ499.
