Presumably if they have apps and the devs use the passbook APIs they'll be in passbook. Think it will take time to filter thru depending how on the ball each company is.
But, like, would stores need anything special to accomadate this, or can any barcode reader scan your phone? It seems that if all stores could use phones for stuff like this, the world would be simpler.
If you are in the UK and have an iOS6 iphone or ipod, you can create a coupon pass for PizzaExpress at passfu.com It looks pretty much like the image below, although my coupon code is different.
You just need to show this code when asking for your bill... as the pass says at the bottom.