If you need to execute the plugin on machine without any svn in the path you can configure the mojo to use the svnjava provider.
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>buildnumber-maven-plugin</artifactId>
<version>1.0-beta-3</version>
<executions>
<execution>
<phase>validate</phase>
<goals>
<goal>create</goal>
</goals>
</execution>
</executions>
<configuration>
<doCheck>true</doCheck>
<doUpdate>true</doUpdate>
<providerImplementations>
<svn>javasvn</svn>
</providerImplementations>
</configuration>
</plugin>
</plugins>
</build>