Unfortunately, unless it was changed in 3.5 & I haven't seen it yet, the "Defending spiked shield" idea won't quite work. According to the 3E FAQ:
"You can’t use the weapon like a shield; if you hold the weapon in your off hand and claim an Armor Class bonus for it, you take all the penalties for fighting with two weapons, even if you don’t actually attack with the weapon."
This leads me to believe that, if you use the Defending ability of the spiked shield, not only do you suffer the penalty for using two weapons, but you also lose your shield bonus to AC, since you're technically "using the shield as a weapon" to gain the Defending AC bonus.