After struggling for a while, I finally got DBUnit and TestNG work with my seam-gen generated project. I use Seam 2.2.0.GA and there are some tips or workrounds, so I list them here for other guys who still suffer from it.
First, you need to modify the puJndiName in components-test.properties:
Change

# These properties are used to replace Ant-style tokens in the component descriptor (components.xml) at runtime.
jndiPattern=#{ejbName}/local
debug=true
seamBootstrapsPu=true
seamEmfRef=#{entityManagerFactory}
puJndiName=#{null}

To

# These properties are used to replace Ant-style tokens in the component descriptor (components.xml) at runtime.
jndiPattern=#{ejbName}/local
debug=true
seamBootstrapsPu=true
seamEmfRef=#{entityManagerFactory}
puJndiName=java:/xxxEntityManagerFactory

Where xxx is your project name. I think this is a seam-gen bug, it puts a null in the expression.
Second, create a XxxTest.xml under test dir where Xxx is your entity, for example Person.

<!DOCTYPE suite SYSTEM "http://testng.org/testng-1.0.dtd" >
<suite name="PersonTestSuite" verbose="2" parallel="false">
    <test name="PersonTest">

        <parameter name="datasourceJndiName" value="java:/DefaultDS"/>
        <parameter name="database" value="HSQL" />

        <classes>
            <class name="com.mydomain.demoproject.test.PersonTest" />
        </classes>

    </test>
</suite>

Note, since the data source in the persistence-test.xml is the DefaultDS, we must use same value here.
Third, create a flat xml data file for DBUnit, say, BaseData.xml under test dir:

<?xml version="1.0" encoding="UTF-8"?>

<dataset>
    <PERSON
        id="1"
        version="1"
        name="A" />
    <PERSON
        id="2"
        version="1"
        name="B" />
    <PERSON
        id="3"
        version="1"
        name="C" />

</dataset>

Fourth, create XXXTest.java, say PersonTest.java:

public class PersonTest extends DBUnitSeamTest {

    @Override
    protected void prepareDBUnitOperations() {
        beforeTestOperations.add(new DataSetOperation("BaseData.xml"));
    }

    @Test
    public void testPersonList() throws Exception {
        new NonFacesRequest("/PersonList.xhtml") {
           @Override
           protected void renderResponse() throws Exception {
               List persons = (List)getValue("#{personList.resultList}");
               Iterator itr = persons.iterator();
               while(itr.hasNext()) {
                   Person person = (Person)itr.next();
                   System.out.println(person.getId()+":"+person.getName());
               }
           }
        }.run();
    }

}

Note, you should care about the path of DataSetOperation. There is a bug in the seam-gen's build.xml, it will not copy the BaseData.xml to the test-build. We will fix it in the next step.
Fifth, modify the buildtest target in the build.xml:
Change

<copy todir="${test.dir}" flatten="true">
            <fileset dir="${src.test.dir}">
                name="**/*Test.xml"/>
            </fileset>
 </copy>

To

<copy todir="${test.dir}" flatten="true">
            <fileset dir="${src.test.dir}">
                    <include name="**/*.xml"/>
            </fileset>
 </copy>

Note, this will copy any xml file to test-build, in this case, BaseData.xml
Sixth, run the test, and you will get:

1:A
2:B
3:C
PASSED: testPersonList

Now, you can list the person data in the DBUnit plat xml file using TestNG test. You can see BaseData.xml is copied to test-build dir with PersonTest.xml file.

Conclusion:

• Pay attention to components-test.properties, persistence-test.xml, there is some exception about jndi name related to this kind of configuration

• Pay attention to build.xml, sometimes it will ignore your configuration file

• Pay attention to resource path in your test code, otherwise DBUnit will throw exceptions like org.dbunit.dataset.DataSetException: java.net.MalformedURLException