I've noticed that when the stock Google Maps app has an update (which hasn't been for a while now) it shows up as "update available" in the Market.
What I've found though, is that some of the other "stock apps" don't do that. For instance, the stock "Facebook" app and the stock "Goggles" app.
When searching through apps in the Market I find updates to those apps, but why aren't they showing up automatically like "Maps"? Am I missing something?