Well, “su” and “sudo” are two completely different things. When you try to run Yast from the launcher “kdesu” is used, which in turn uses “su”.
What happens if you try to run “su” in a terminal window?
Is this a fresh installation or an update from an earlier milestone?
su was moved from coreutils to util-linux. If you updated but skipped a milestone, this migration possibly wasn’t done correctly.
See this bugreport, maybe you find help there: https://bugzilla.novell.com/show_bug.cgi?id=827845
It should work fine on a fresh beta1 install though (and it does here).