It's mostly considered declasse' to simply refer to somebody else's post on some subject. And I don't do so very often, but this post on writeable CTEs is too much to pass up, not least because I've yet to move to 9.1, and won't likely until it hits 9.1.3 (currently at 9.1.1). What makes the post of interest: Brown states that PG is alone with wCTE, and some quality time with searching supports that. My interest rests on the motivation for this endeavor: put, and keep, the logic in the datastore and let the client pretty-pretty the screens as it wishes. Writeable CTEs take this notion yet another step forward.
Not that I'd choose a joined table with a calculated, by code, column. But that's another story. I'd be more inclined to implement an inventory/order line update, since that fits with the canonical example of join. For example, if your customers are mixed with regard to price changes, some will get updated prices on unfilled order lines, the rest not. In that circumstance, you'd want to update some order lines when the inventory (or price break) table is updated. That sort of thing. 9.1.3 shouldn't be too far away.
Good on 'ya, as Texans say.