The Player's Handbook says that shield spikes cannot be placed on a tower shield. The reason behind this appears to be the inability of characters to use a tower shield to do a shield bash. Although this line of logic is sound, shield spikes may have other uses other than the shield bash.
A group of characters I was DMing formed a shield wall to keep their oppenents away while range fighters weakened down their fighters. The shield wall was broken when a raging barbarian did a bull rush manouver against one of the ends of the wall. One of the characters made a remark about placing spikes on the shields to make the rushing characters take damage. This was actually a good idea.
When shield spikes are placed on a tower shield (a character may place extra spikes on the shield to provide extra protection but this option gives an extra -1 Armor Check Penalty), they are not considered weapons because they can't be used offensively. When bracing the tower shield, the character can hold the spikes against a charge in a similar manner to someone setting a spear. The spikes do not do extra damage for being set against a charge but they do 1d6 damage (or 1d8 depending on the number of spikes) against the charging opponent. The opponent may attempt a reflex save (DC 15~20) to avoid the damage.
This doesn't provide the user of a tower shield any extra bonuses against charge and bull rush actions. An attacker must decide if it is worth the pain to push his way through a "spiked barrier" to get to his target. Having a spiked shield also has other logistical problems. Carrying around a large, spiked piece of steel or wood may cost a character more than he bargains for. Carrying wounded characters on these shields no longer seems like a viable option.