The principal value of the shield is in making secondary attacks miss, so it gets better as you go up in levels and numbers of attacks in a full attack increase.
Viable? Absolutely! Will it make you the Super-star of the party... naw, probably not.
I had a Dwarven Fighter 2 / Rog 3 who wore Fullplate and used a Towershield and Waraxe... he was constantly shown up in the damage department by the Dwarven Barbarian 1 / Fighter 3... but damn my Fi/Rog was hard to hit.
More than viable, the armored fighter using sword and shield is one of the strongest archetypes of any medieval like combat and so, it is pretty cool in itself. I haven't read the PHB2 and so I cannot comment about the shield feats there but I hope they are really good, as shields in D&D are much less effective then they actually are in real life.
Shield Specialization (PHBII) +1 AC
Heavy Armor Optimization (Races of Stone) 4th level +1 AC
Greater Heavy Armor Optimization (Races of Stone) 8th Level +1 AC
Armor Specialization (PHBII) 12th level DR 2/- with chosen armor type
Full Plate + Heavy Shield + 12 Dex
8 + 1 +3 + 2 = 24 AC before magic at 12th level, and DR 2/-
At high levels? I think the "buckler while wielding a weapon two handed" rule sort of gives the edge to the two handed fighter, because enchantments start to come into play at higher levels and a buckler works as well as a full size shield for enchantment.
Which is true, though you do not get any benefit from a buckler when you attack, and you take a -1 penalty when you attack while using a buckler.
The question is not is Sword and Board better than any other style, but rather is it viable. That answer is yes.
Now if you want to ask is Sword and Board better......
Well I have a 12th level Fighter running around in a Dawnforge game that is Sword and Board. He is beginning a switch to an Angel's Blade (Racial Greatsword) due to his receiving it from a dead king. If not for that RP moment, I would have continued Sword & Board.