I guess that would depend on the type of graphics used in the apps. If it's mostly just text, then the text should scale without any pixelation at all as it is somewhat of a vector format.
Originally Posted by Calot
I've never developed apps for a mobile device, but I do a lot of graphics work in both a raster and vector environment. From what I've seen of WP it is largely based on typography, so the text should scale very well.
Obviously there will situations where developers have created custom graphics for the lower resolutions, and those may take a little bit of time to be updated.