I'm using like this now:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>buildnumber-maven-plugin</artifactId>
<version>1.2</version>
<executions>
<execution>
<phase>validate</phase>
<goals>
<goal>create</goal>
</goals>
</execution>
</executions>
<configuration>
<format>{0} - {1,date,yyyy-MM-dd HH:mm:ss}</format>
<items>
<item>scmVersion</item>
<item>timestamp</item>
</items>
</configuration>
And access the value from jsp like this:
${initParam['build']}
This shows the SVN revision number first and then the timestamp.