Interesting article at NYTimes.com by Alan Schwarz(free registration required) about the value of a walk. I'm not sure how the calculations should happen, but the point is that walks should be weighted differently based on the batter.
For example, if your pitcher walks Barry Bonds, you wouldn't feel so bad about that. But, if your pitcher walks the 9 batter (pitcher), you're ready to cut your pitcher.
So, when calculating OBP or OPS, shouldn't walks count differently based on the type of hitter? Good question, and some interesting fodder for discussion among SABR-ites.