Question

Update: moved the solution to an answer

Eclipse Juno SR1 installs m2e plugin version 1.2. The m2e connecter buildhelper provided by the Eclipse marketplace is not compatible with this version of the m2e plugin. I’ve been looking through various mailinglists, but I cannot find a location where to find an updated buildhelper.

The following error occurs when trying to install the m2e connector buildhelper:

Operation details
Cannot complete the install because of a conflicting dependency.
Software being installed: m2e connector for build-helper-maven-plugin 0.15.0.201109282249 (org.sonatype.m2e.buildhelper.feature.feature.group 0.15.0.201109282249)
Software currently installed: m2e - Maven Integration for Eclipse 1.2.0.20120903-1050     (org.eclipse.m2e.feature.feature.group 1.2.0.20120903-1050)
Only one of the following can be installed at once: 
    Maven Integration for Eclipse JDT 1.2.0.20120903-1050 (org.eclipse.m2e.jdt 1.2.0.20120903-1050)
    Maven Integration for Eclipse JDT 1.1.0.20120530-0009 (org.eclipse.m2e.jdt 1.1.0.20120530-0009)
Cannot satisfy dependency:
    From: m2e - Maven Integration for Eclipse 1.2.0.20120903-1050 (org.eclipse.m2e.feature.feature.group 1.2.0.20120903-1050)
    To: org.eclipse.m2e.jdt [1.2.0.20120903-1050]
Cannot satisfy dependency:
    From: m2e connector for build-helper-maven-plugin 0.15.0.201109282249 (org.sonatype.m2e.buildhelper 0.15.0.201109282249)
    To: bundle org.eclipse.m2e.jdt [1.1.0,1.2.0)
Cannot satisfy dependency:
    From: m2e connector for build-helper-maven-plugin 0.15.0.201109282249 (org.sonatype.m2e.buildhelper.feature.feature.group 0.15.0.201109282249)
    To: org.sonatype.m2e.buildhelper [0.15.0.201109282249]

In our pom files we use buildhelper to add directories with generated sources to the Eclipse:

<plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <version>1.5</version>
    <executions>
      <!-- Fix this eclipse error by discovering the plugin in the marketplace -->
        <execution>
            <id>add-source</id>
            <phase>generate-sources</phase>
            <goals>
                <goal>add-source</goal>
            </goals>
            <configuration>
                <sources>
                        <source>${project.build.directory}/generated-sources/cxf</source>
                </sources>
            </configuration>
        </execution>
    </executions>
</plugin>

Answer 1

The comment in the pom suggested to use the Eclipse marketplace to install the buildhelper:

<!-- Fix this eclipse error by discovering the plugin in the marketplace -->

This will not do, because the buildhelper version in the Eclipse marketplace is outdated.

Instead of using the Eclipse Marketplace, I installed the buildhelper directly from the Sonatype repository using the Eclipse install new software menu option. The Sonatype repository with the (currently) latest version is here:
https://repository.sonatype.org/content/repositories/forge-sites/m2e-extras/0.15.0/N/0.15.0.201206251206/

This fixed my problem.