Last week Google released an update to the Gmail app for Android that added new functions to control how much data synchronizes with Android phones. Unfortunately, it appears that the update removes some API functions that third party apps relied upon, causing those apps to no longer work.
The change that Google has made prevents apps other than Gmail to access the Gmail database. Previously, apps only had to declare that they use the read Gmail permission and they were allowed access.
TechCrunch has a list of six apps that are known to be affected, although there could be several more. My guess is that the most widely used of the affected apps is LauncherPro, specifically the LauncherPro Plus Gmail widget and LauncherPro Gmail dock notifications stop working after the Gmail update is installed.
The LauncherPro issue described above is typical of the type of apps that will be affected, specifically any widget or app that accesses the Gmail DB to provide notifications of new email, or for that matter presents e-mail.
According to TechCrunch, Google made the change because of potential security issues. At the moment it is not known whether Google will be providing a different, presumably more secure, way to access the Gmail database.