There are a lot of things wrong with this error message. First of all, it gives me absolutely no idea which app is not available. Second, I had asked iTunes which apps needed to be updated, and it told me that this Mystery App was out of date. How can it be out of date if it isn't available at all? Third, after telling me that this one application wasn't available, iTunes failed to update the other 60+ applications.
The prospect of manually updating them all was fairly annoying. As I looked through all of the apps shown on the Update page, I realized that one of the apps didn't have the "Update Now" button next to it, and deduced that was the app causing the problem. I went into the main list of apps, control-clicked on it, and then chose "Show in Finder." I moved the app from my iTunes folder to my Desktop, and then deleted the app from iTunes, and ran "Check for Updates" again. It showed one fewer app needing to be updated, and the Update All process went off without a hitch.
If you find yourself facing this fairly unhelpful message from iTunes, look through the list of apps-in-need-of-updating to see if any of them lack the Update Now button. It should stand out from all of the others.
I find it frustrating that iTunes continues to not provide basic user feedback, such as telling me which 'item' it was referring to in the error message above. iTunes is the link between Apple's highly profitable iPod, iPhone, and iPad devices, not to mention perhaps the most important Windows crossover app that Apple produces. It should be one of Apple's best applications, if not the best. Instead, it is the one I least enjoy and which shows the most rough corners. I hope that, between now and iTunes 11, we'll see a whole lot of improvements and spend less time tinkering around with the UI.