Several of the big voltage-regulator manufacturers (National, Linear, ST come to mind) have software along these lines already, some of which is fairly good. A unified and less cumbersome application would be great, especially if it could pick out passives (caps, as you suggest, but also inductors and freewheel diodes for switchers). National's Webench already does this, but its library of passives is very limited compared to Octopart and many of the parts it suggests are either expensive or hard to find in low quantities.