Surprisingly, It does not give an increase to the damage given to units. It gives archers and ships a multiplier against ships and buildings though.
In case you ever want to find out what an upgrade is doing to your unit, click your unit, press f1, and look at the upgrades. Mouse over the one that you are concerned about, and look at the bonuses.
Thank God SCII finally has a release date but it so so damn far away...
ESO Name: AoTH_ZoOkEePeR [This message has been edited by Ferral (edited 09-10-2007 @ 00:52 AM).]