Over/under invoice is very dependent on the dealers inventory. If they don't have the Z on the lot, it's gonna be tough to negotiate to invoice. If they have 20 Zs on the lot, a few hundred below invoice shouldn't be hard this time of year.
For 2010, I'd expect a lot of dealers to stand firm between sticker and invoice for a few months before inching towards invoice mid-year.
Of course, I may also have no idea what I'm talking about. You've been warned!!