The SRD linked in the question actually omits all mention of shields in the first paragraph where it discusses the effects of the "masterwork quality" inherent in adamantine-based items. This would seem to indicate that the lack of any explicit benefit for adamantine shields could very well be an oversight rather than an intentional omission, since at the very least adamantine shields should be considered masterwork and thus have a reduction in armor check penalties.
As was pointed, the optional armor rules make clear that the AC bonus for shields result from deflecting attacks. As a result, a failure to block an attack while wielding a shield means that the blow "connects" despite the attempted block.
This can be interpreted in two ways:
- The shield bearer totally misjudged the attack and the attack "got around" the shield.
- The shield didn't deflect the attack and force of the weapon clashing against the shield causes damage to the bearer.
Now while 1 makes sense when dealing with something small like a buckler or when facing projectile fire, 2 is much more likely when applied to a heavy shield in melee.
The adamantine breastplate has both damage reduction (over and above the optional rules linked above) and AC bonus, even though one might argue that a "hit" might be a blow to the arms, head or legs, none of which are protected by a breastplate. Implicitly we're assuming that the blow impacts but is mitigated by the adamantine within the armor in the event that the AC bonus of the armor doesn't "stop" the attack.
Applying that same logic in this case would give us option 2 - a standard metal or wooden shield doesn't provide protection from a "direct" blow that connects with the shield. But this would mean that some sort of DR should apply if the shield itself is made of adamantine. Tentatively, I would suggest that the SRD providing shields categorized as "light" and "heavy" in the armor table would mean that they should be treated as armor of that type for determining DR from their special materials.
A shield already provides as much AC as leather armor - what you gain in an "easy AC bonus" you sacrifice for not being able to wield two handed weapons, bows, or just having a free hand in combat. If the material provides inherent damage reduction when incorporated into armor it is NOT an imbalance to have that same reduction apply to a shield, since the tradeoffs of one vs the other is already incorporated into the ruleset.