We had to bite the bullet here and add an extra word to every thunk,
to enable running ordinary libraries on SMP.  Otherwise, we would have
needed to ship an extra set of libraries with GHC 6.6 in addition to
the two sets we already ship (normal + profiled), and all Cabal
packages would have to be compiled for SMP too.  We decided it best
just to take the hit now, making SMP easily accessible to everyone in
GHC 6.6.

Incedentally, although this increases allocation by around 12% on
average, the performance hit is around 5%, and much less if your inner
loop doesn't use any laziness.