It happens to us all: an application update for the iPhone or iPad goes awry, and the new app doesn't do things the way we want (or, unfortunately, work at all on one of our devices). We ourselves are in that boat at the moment, since the new version 2.0 of the TUAW iPhone app is not working as expected on the iPad -- our development team is digging into the problem right now and we hope to be fast-tracking a fixed version right away. Our apologies for the issues.
This sort of thing is particularly sticky because there's no Apple-supported way to roll back to a previous version of an iOS application once you've updated. There is a workaround, however, if you haven't yet emptied your Trash in the Finder (or if you can restore your Mobile Applications folder using Time Machine). Read on for the steps.