This is the very first time I report something here so I really hope it is the good place.
I make a lot of character developement and now work nearly exclusively with cycles.
Since map baking is available with Cycles, normal maps look way better with this engine, more contrasted and accurate but...
whenever you try to bake a normal map from a multiresolution mesh with subdivide UVs option checked in the multires modifier.
Assuming you'll use margin to cover the UVs islands corners, Cycles is not able to interpolate these pixel correctly and create darken or transparent area on the bleeding pixels.
This generate seems when you then use these normal maps on your object.
If subdive UVs is desactivated, the margin works just fine.
Please let me know if you have any needs of exemples for this and if this is an already reported problem.
In anycase, thanks for your interest.