A player in a game I ran yesterday spontaneously asked if he could have spikes on his armour and shield. My response was "erm, I guess so", and that was that. He used the spikes several times during the adventure, including dealing automatic damage to opponents who were hitting him. It was all good fun, and it was only a one-shot, so I didn't feel it necessary to come up with any negatives to balance out the obvious benefits he was getting.
Having now had some time to consider it, I think something like the following might work as a set of rules for spiked armour.
Banded, splint or plate mail can be forged with integrated spikes. This increases the cost of the armour by 50%. Either small or large spikes can be chosen. Small spikes increase the weight of the armour by 5lbs and deal 1d3 damage. Large spikes weigh an additional 10lbs and deal 1d6 damage.
The character can make damaging attacks essentially unarmed, by simply throwing himself at opponents in an attempt to impale them on his spikes.
When hit by an opponent using natural weapons (claws, fists, bite, etc.) the character's spikes have a chance of inflicting damage on the attacker -- 10% per point of the attacker's AC. This chance applies per round against foes with a grappling, grabbing, swallowing or constricting attack.*
On a natural to-hit roll of 1 the character spikes himself, suffering damage from the spikes. This is in addition to the normal effects of rolling a 1.
On taking falling damage the character is also damaged by the spikes on his armour. There is also has a 10% chance per 10' of distance fallen that the spikes on the armour are destroyed.
In most civilised regions such armour is regarded as dangerous, vulgar and brutal -- a sure sign of someone who's looking to start a fight. The character may get into trouble with the law if he wears spiked armour in towns and cities.
Indulging in a nice heroic embrace with ones companions after a battle is not recommended. The spiked character may risk becoming a social outcast.
A shield covered in spikes can also be purchased, again with either small or large spikes. A shield with small spikes costs 20gp and weighs 15lbs. A shield with large spikes costs 30gp and weighs 20lbs. Spiked shields have none of the benefits or drawbacks of spiked armour, except perhaps the drawback of being regarded by law-keepers as a sign of a dangerous rogue. A spiked shield can be used to make melee attacks, dealing damage based on the size of the spikes. If an optional rule for two-weapon fighting is used, a spiked shield may be used to make an off-hand attack. When using a shield offensively it grants no AC bonus.
* Lambton worm yeah!