When Maven is running in offline mode (set in the Maven section of the Eclipse preferences), if you haven’t specified a version for certain plugins — like the maven-bundle-plugin — Eclipse will report an error like this:

Project build error:Cannot resolve pre-scanned plugin artifact (for use as an extension): org.apache.felix:maven-bundle-plugin: Failed to resolve extension plugin: org.apache.felix:maven-bundle-plugin:maven-plugin:RELEASE

Maven outside of Eclipse will most likely continue to work fine, but Eclipse will be one unhappy camper.  Your project may even lose its Java nature if m2eclipse is unable to resolve the dependencies.

The solution is to either allow Maven to run in online mode (Preferences > Maven) or add a <version> element to the plugin configuration (see example below). It seems that specifying the version in the pluginManagement section of the pom.xml may not even solve the problem.

<plugin>
	<groupId>org.apache.felix</groupId>
	<artifactId>maven-bundle-plugin</artifactId>
	<version>1.4.3</version>
	<extensions>true</extensions>
	<configuration>
		...
	</configuration>
</plugin>

Once you make one of these changes Eclipse/m2eclipse should rescan the project and fix it.  If it doesn’t get fixed immediately, right-click on your project in the Package Explorer and choose Maven > Update Dependencies and Maven > Update Project Configuration.

Note: I’m assuming you’re using a stable development build of m2eclipse. The non-dev version didn’t have these options when I last checked.