The early 2011 MBPs with Thunderbolt came with
Mac OS X 10.6 Snow Leopard, the late 2011 MBPs came with
Mac OS X 10.7 Lion, though as the Thunderbolt chip in both 2011 editions of the MBP was the same, one can run
Mac OS X 10.6 Snow Leopard even on late 2011 MBPs.
The 2011 MBA has a different Thunderbolt chip (smaller, due to space constraints), for which there are no
Mac OS X 10.6 Snow Leopard drivers.
Maybe someone found a workaround, thus my link to
MRoogle.