Raising minimum iOS Deployment Target Version for App Update

From my experience those updates just won't show up as available.

When I upgraded OS on my device from 3.1 to 4.1 about 10 available updates appeared immediately in App store app - so that should be the actual behavior.

In addition to only showing supported updates, the store now offers the "last compatible version". This lets people download an app even if their device doesn't support the most recent version. Unfortunately this means that some people could still download an older version with bugs you have already fixed. There may be a way to disable this, but none of my app updates have introduced new requirements, so I can not test.

It's nearly a safe bet that they won't be allowed to install it. A similar situation is iPad apps or Mac apps which won't display in the App Store on iPhones and iPods.

I say nearly because the updates should not appear to older users on their iOS devices. The risk, however, is when users sync with iTunes, or if they update with another device. The new version of the app is now associated with their account, and will ruin the install on the older device if they try to sync it with iTunes.