Apple has fixed a glitch that caused some.
In a statement issued to AllThingsD, Apple confirms that the problem was traced down to a fault with "a server that generated DRM code for some apps being downloaded".
According to the statement, only a small number of users were affected.
The DRM code normally prevents apps being run on unauthorized devices, but in this case it seems the glitch prevented the app from working on legitimate devices.
"The issue has been rectified and we don’t expect it to occur again," Apple added.
All affected apps will have to be deleted and re-downloaded from the appropriate App Store. This is an annoyance because it will mean losing all settings and data inputted into the app but it will fix the problem.
Instapaper creator Marco Arment reported yesterday that he was "deluged by support emails and Twitter messages from customers," who could no longer launch his app following the release of the new 4.2.3 update.
"Lots of anxiety and research led me to the problem: a seemingly corrupt update being distributed by the App Store in many or possibly all regions," writes Arment on his personal blog.
The issue affected a whole range of apps, including hit titles such as Angry Birds Space HD Free.
UPDATE: I'm getting reports that that Apple has tweaked its App Stores to allow corrupted updates to appear and out of date to allow updates to be pushed to affected users. This is good because it means that users don't have to delete old apps and won't lose settings and data.