Maven release:prepare : Cannot prepare the release because you have local modifications

I can confirm, that adding the plugin or configuring it to exclude the pom.xml check did work:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-release-plugin</artifactId>
  <configuration>
    <checkModificationExcludes>
        <checkModificationExclude>pom.xml</checkModificationExclude>
    </checkModificationExcludes>
  </configuration>
</plugin>

Removing my project's target folder from source control fixed this issue for me.

For everybody who don't want to change the pom.xml a commandline option does the trick:

mvn [...] -DcheckModificationExcludeList=pom.xml,.maven/spy.log

I had the problem on our hudson / jenkins CI environment which complained about changes in the .maven/spy.log