Here in Brazil we use dry yeast a lot more than liquid 'cause usually it's tough to import liquid yeast from US, so when some BrewShops get some, they sell pretty fast.
Chris White and Jamil on Yeast book say that you can kill a lot of cells (book says up to 50%) if you do not re-hydrate with water because on the first moments yeast cannot regulate its cell wall permeability, so besides osmosis shock they say things like hop acids and other stuff might get in the cell and kill it.
While researching about this subject I found a very nice experiment from a Homebrewer:
http://www.youtube.com/watch?feature=player_embedded&v=vOrfmzpDmPkBasically, he splits the same wort in 4 different fermentors and he compares the same dry yeast with and without re-hydrating them using wort without aeration, and then the same thing aerating the wort with pure O².
The result is that lag-time on re-hydrated yeast was much shorter, and O² aeration pretty much didn't make any difference, which corroborates to some theories about high reservers of sterols and fatty acids dry yeast has due they condition the yeast before lyophilization (cooling the yeast gently slowly, trehalose, etc), which (in theory) makes aeration not so much important for regular OG worts (1050) with the proper amount of re-hydrated dry yeast.
On my batches, I observe exactly the same behavior as the experiment.
Regards,
Guenther