Desabilitar submit sino hay ningun checkbox seleccionado

[KarlanKas]

Te sirve esto?

Código HTML:

<script>
function pepe(){
f=document.forms['formul'];
f.elements['action'].disabled=true;
for(a=0;a<f.elements.length;a++){
if(f.elements[a].type=="checkbox" && f.elements[a].checked==true){
f.elements['action'].disabled=false;
break;

}
}

}
</script>
<form name="formul" action="loquesea.php">

<input type="checkbox" id="1" name="fichier0" value="11" onclick="pepe()" />
<input type="checkbox" id="2" name="fichier1" value="algo" onclick="pepe()" />
<input type="checkbox" id="3" name="fichier2" value="fotos" onclick="pepe()" />
<input type="submit" name="action" disabled="disabled" value="Enviar" />

</form>