Particle_Man said:
re: dex bonus. So if someone has a +4 or greater dex bonus that means that they will never accidentally be hit as cover because they can dodge out of the way?
It depends on the relative Armor Classes of the two characters. In general, the covering character needs to have a better AC than the intended target (prior to the cover bonus) for this to come into play.
For example, an AC 18 character (including his +4 AC bonus for Dexterity) is giving cover to an AC 20 character. With the cover bonus, the intended target has an AC of 24. On a roll of 20-23, the covering AC 18 character is hit. His Dexterity doesn't enter into the equation.
If the intended target was only AC 17, on the other hand, then his AC with cover is 21. On a roll of 18-20, the covering character is hit; but on a roll of 17, the covering character isn't hit (because that doesn't hit his AC 18), and since that would be good enough to hit him but for his Dex bonus, the shot actually hits the intended target after all. (Yes, the archer got a better result from rolling lower.)
Finally, suppose the intended target was only AC 15, and the covering character uses his Dodge feat to get a +1 Dodge bonus to AC against attacks from his archer ally. (Why? Wouldn't you, if your archer ally liked to shoot past you into melee?) Now we get a fun situation! The target has AC 19 with his cover bonus-a roll of 19 or more hits him. A roll of 15-18 would hit the covering character--but he has AC 19, so it doesn't. Furthermore, since that character has only AC 14 without his Dex and Dodge bonuses, those attacks actually do hit the 'covered' character-he doesn't get any effective cover in this situation.
And that, boys and girls, is probably why they made this an optional rule in v. 3.5 ... as Patryn so ably noted.