In addition to bguiz' answer, which would be the best solution, I've created a workaround using Maven profiles, bypassing the problem.
This is a temporary solution, until the maven-exec-plugin's bug gets fixed.
Please upvote the bug report here: http://jira.codehaus.org/browse/MEXEC-118
Edit: The bug is resolved, you can point to 1.4-SNAPSHOT to fix it.
<project>
(...)
<profiles>
<profile>
<id>grunt-exec-windows</id>
<activation>
<os>
<family>Windows</family>
</os>
</activation>
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>${exec-maven-plugin.version}</version>
<executions>
<execution>
<id>grunt-default</id>
<phase>generate-resources</phase>
<configuration>
<executable>cmd</executable>
<arguments>
<argument>/C</argument>
<argument>grunt</argument>
</arguments>
</configuration>
<goals>
<goal>exec</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</profile>
</profiles>
</project>