Sorry. Before you run "mvn install" you should make a few modification in pom.xml.

geronimo-3.0.0\assemblies\pom.xml
Line83-85
                <!--<module>geronimo-jetty8-minimal</module>
                <module>geronimo-jetty8-javaee6-web</module>
                <module>geronimo-jetty8-javaee6</module>-->
to
                <module>geronimo-jetty8-minimal</module>
                <module>geronimo-jetty8-javaee6-web</module>
                <module>geronimo-jetty8-javaee6</module>

geronimo-3.0.0\assemblies\geronimo-jetty8-javaee6\pom.xml
Line28
        <version>3.0.0-SNAPSHOT</version>
to
        <version>3.0.0</version>

geronimo-3.0.0\assemblies\geronimo-jetty8-javaee6-web\pom.xml
Line28
        <version>3.0.0-SNAPSHOT</version>
to
        <version>3.0.0</version>

geronimo-3.0.0\assemblies\geronimo-jetty8-minimal\pom.xml
Line28
        <version>3.0.0-SNAPSHOT</version>
to
        <version>3.0.0</version>


That is all.
Please try it.
I have builded the geronimo server with jetty successfully.

geronimo-jetty8-javaee6-3.0.0-bin.zip
geronimo-jetty8-javaee6-3.0.0-bin.tar.gz
geronimo-jetty8-javaee6-web-3.0.0-bin.zip
geronimo-jetty8-javaee6-web-3.0.0-bin.tar.gz
geronimo-jetty8-minimal-3.0.0-bin.zip
geronimo-jetty8-minimal-3.0.0-bin.tar.gz



2013/3/6 Zhi Xie <daxiezhi@gmail.com>
You can build the server by yourself. 
For example, 
1. download geronimo 3.0.0 from the SVN server (https://svn.apache.org/repos/asf/geronimo/server/tags/geronimo-3.0.0). 
2. run "mvn install" or "mvn clean install" under the root dir(geronimo-3.0.0). 
3. When the build is finished successfully, you can get the geronimo3.0.0 server with jetty in assemblies directory(geronimo-3.0.0/assemblies).



--
Best Regards
Gary