@DRD1812: It's our house rule, but I know that we picked it up somewhere on these forums.
Summarily: A shield and light armor would give a 4+ save. This applies to shooting, too.
Our "6+, regardless, if the target is in front," stems from wanting to make carrying a shield a great boon. Mathematically (wasn't there a thread about this?), two models, identical in all aspects sans armament (one has 2 weapons, one has weapon and shield) will see the one dual-wielding winning more often.
Additionally, if the save roll is successful, and the particular attacker a large creature, we rule that the shield is destroyed.