That will depend on individual apps.
For example. I have a banking app that fits on the HTC Desire (Original). If fits the screen perfectly.
Then I moved to the HTC Sensation with a bigger screen and many more pixels (NICE) and the app remained the same size. That means the app kept it's size ratio and did not take up the whole screen. But that is Gingerbread and ICS might have something similar to Honeycomb to battle this.
most apps look fine... all the google apps look great (obviously), but i noticed that in a couple (like fruit ninja), you can definitely notice that they are not 720p... they still look fine, but hopefully app makers will update
If you are worried about a specific set of apps, I'm more than willing to test for you. I have 3 different Android devices at 3 different resolutions (an Optimus V, a Nexus One, and a Galaxy Nexus).
If you are worried about a specific set of apps, I'm more than willing to test for you. I have 3 different Android devices at 3 different resolutions (an Optimus V, a Nexus One, and a Galaxy Nexus).