I've wondered the same thing, but according to this (he's written a couple of the most popular books on bike maintenance), it's fine: http://velonews.com/article/89649/velonews––resident-techno-guru-takes-your-questions
Dear Lennard,
I am considering storing my new full suspension bike using a vertical type rack that stores a single bike hanging vertically from the wheel.
I was wondering if that could cause any damage to either suspension, front or rear.
Does it make sense to hang the bike from the rear wheel?
Nir
Dear Nir,
Hanging it from the front or rear wheel will not hurt it unless either of your shocks leak oil through the upper seals. In that case, hanging it could still be a good thing, because it will alert you to the problem and motivate you to fix it.
Lennard