I've blogged about co-op before, mes auteurs, but I haven't really touched on one important aspect of the co-op system: the promotional quantity.
A promotional quantity is the number of copies a store or chain needs to take in order for them to have enough to put the book into co-op placement. This number varies widely depending on the retailer: an independent book store might only need a couple dozen copies in order to put a title front-of-store, whereas a chain will need to buy several thousand. Essentially, you need enough copies to make a sizable display somewhere in the store (or in every store, assuming placement at a national chain) for at least a couple of weeks.
Without quoting numbers or otherwise divulging non-public information, I can tell you that unless your announced initial print run is roughly in the mid- to upper tens of thousands (as you may or may not know, announced first prints are always higher than actual first prints) or higher, it's unlikely that your book will get a large enough buy at a major retailer to ensure national co-op placement. You might get placement at a few independent retailers, but as I've mentioned before, their co-op programs are much less rigid than those found in the national accounts, and (unfortunately) due to their smaller size, their influence is limited.
It would be a mistake, however, to underestimate the grassroots/guerilla/word-of-mouth campaigns that independent book store owners and employees are capable of waging. LibraryThing, Twitter, Facebook, and the blogosphere at large are all arenas in which the indie store can be king: recommendations from knowledgeable, non-corporate industry insiders can go a long way, and I've seen books take hold at the independent store level and work their way up to become national bestsellers. Your book might get skipped at a major chain, get buzzed by a handful of independent booksellers, and end up getting ordered at a promotional quantity by that same chain when the paperback comes out. Opportunities abound, meine Autoren, and you need to be ready to take advantage of them.
So: whether your initial print run is 10,000 or 100,000, befriend your local independent retailers; pimp yourself and your novel on-line, in person, at readings, and at conferences; and don't get discouraged by the myriad setbacks you'll no doubt encounter.