Kimi will have number "0" if Schumacher and Ferrari win both championships.
Scenarios are thus (I think)

:-
Scenario 1
Alonso wins WDC, Renault win WCC
2007 Renault car numbers 3&4
2007 Ferrari car numbers 5&6
2007 McLaren car numbers 1&2
Scenario 2
Alonso wins WDC, Ferrari win WCC
2007 Renault car numbers 5&6
2007 Ferrari car numbers 3&4
2007 McLaren car numbers 1&2
Scenario 3
Schumacher wins WDC, Renault win WCC
2007 Renault car numbers 0&2
2007 Ferrari car numbers 3&4
2007 McLaren car numbers 5&6
Scenario 4
Schumacher wins WDC, Ferrari win WCC
2007 Renault car numbers 3&4
2007 Ferrari car numbers 0&2
2007 McLaren car numbers 5&6