I've just installed a brand new Joomla 1.5 version on my site, with the latest CK Forms version (no other components or plugins installed but core Joomla and CK). The form works fine, but doesn't validate any of the fields set to mandatory, meaning you can submit a completely empty form. This happens in both Firefox and IE 7.
(Odd thing is that I installed an older version of CK on a different site and had no problems. Installing this old version initially on the first site, where I'm having issues with validation, also caused empty forms to be submitted. So this appears site specific)