I'm using master.
I'm attaching an example file, basically the default cube. In weight painting mode I've set the strength of the brush to 1 and no falloff in the curve. If you paint a vertex with subsurf OFF, you'll see that it behaves as expected and the vertex is assigned a weight of 1.0.
The problem comes when you enable subsurf, paint one vertex and you will notice that the assigned weight is not 1.0, but it seems to be somehow divided by the level of subsurf! If you paint with a subsurf level of 1, the weight is 0.5, with a level of 2 it is 0.333 and so on.
If this is some kind of new feature, it should definitely have a toggle! :D