Update:  MSDN documentation here: http://msdn2.microsoft.com/library/ms181247(en-us,vs.80).aspx 

"Your mileage may vary" on this tip - in fact I can practically guarantee it won't work for you if you are still on Beta 2 like you probably are.  But for those bracing themselves for the July CTP I'm hoping the fixes to make this work will have made it in time.  At the very least just looking at this might give you some insight into what it takes to move VSS 6.0 over to TFS.  

First of course you need your own VSS 6.0 server (in this example, the VSS 6.0 server was named ADWORKS)

1) Install VSS 2005 on the client machine that has VSTS installed - yes you heard me right - VSS 2005.  This is only required because conversion from 6.0 to TFS requires dependency dll's that are in VSS 2005, we will not actually be using VSS 2005 for anything else, we wont' be porting VSS 6.0 to VSS 2005 and then to TFS, we will go straight from 6.0 to TFS.  But vssconverter.exe has some dependencies on dlls that are only installed with VSS 2005.  In my personal opinion this is totally lame, if you think so too send me some feedback - the voice of the people tends to give credibility to my bug reports. 
2) Copy analyze.xml and migrate.xml to a temporary folder (er, create these text files using the templates below)
3) Edit the files in the temporary folder as follows:
a) in both files change any reference to the adworks server name to the server name hosting your VSS 6.0 database
b) in migrate.xml change the __XXX__ variables to their proper values
4) Open a VS Command prompt (Start->All Programs->Microsoft Visual Studio->Microsoft Visual Studio Tools->Visual Studio 2005 Command prompt) and CD to the temp folder you created in step #2

Finally, you are ready for the one-two punch that will move everything over:
5) vssconverter analyze analyze.xml 
6) vssconverter migrate migrate.xml

At this point if you open the team project in the IDE you should see the source code in Team Explorer, double click the node called "Source Control" and Source Control Explorer will open up with all the migrated files.  If this isn't the case, contact me so I can go to bat for you over here. Ahh... Living la vida beta.

Here are the contents of the two XML files:
[analyze.xml]

<?xml version="1.0" encoding="utf-8"?>
<SourceControlConverter>
      <ConverterSpecificSetting>
            <Source name="VSS">
                  <VSSDatabase name="\\adworks\VSS\AdventureWorks"></VSSDatabase>
            </Source>
            <ProjectMap>
                  <Project Source="$/" ></Project>
            </ProjectMap>
      </ConverterSpecificSetting>
      <Settings>
                    <OutputFile>AnalysisReport1.xml</OutputFile>
      </Settings>
</SourceControlConverter>

[migrate.xml]
<?xml version="1.0" encoding="utf-8"?>
<SourceControlConverter>
      <ConverterSpecificSetting>
            <Source name="VSS">
                  <VSSDatabase name="\\adworks\VSS\AdventureWorks"></VSSDatabase>
        <!-- <UserMap name="UserMap.xml" /> -->
        <!--<SQL Server=".\SQLExpress" /> -->
            </Source>
            <ProjectMap>
                  <Project Source="$/" Destination="$/__TEAM_PROJECT__/"></Project>
            </ProjectMap>
      </ConverterSpecificSetting>
      <Settings>
          <TeamFoundationServer name="__APP_TIER__" port="8080" protocol="http"></TeamFoundationServer>
          <OutputFile>MigrateReport1.xml</OutputFile>
      </Settings>
</SourceControlConverter>