In the PHB, the Darkwood Shield is described as "...a large non-magical wooden shield." A regular large shield weighs 10lbs, or twice the Darkwood. If it were a small shield or buckler (which there's no reason it couldn't), then the weight of those should drop accordingly; from 5lbs to maybe 3lbs. Otherwise, I'd say you were right: In the absence of a contridiction, I should assume there is no change in the spell failure statistic. Thanks.
And by the way, Crothian, referring to a seperate thread: If your posts were not thoughtful and well-received, I don't believe you would have reached the 8,000 mark! So there!