Here is a quote from Steinberg.net
So I'm hoping that I will be able to use my uad plugins for sidechaining.The way it will work is that Steinberg will specify how plugin vendors can implement sidechaining as part of the VST 3.0 spec. When that's released, the vendors will all have to write/test/release new versions of their plugins.
So at least initially, only the included Steinberg plugins will have it.
If you were hoping that when 4.1 comes out you'll suddenly be able to sidechain with all your existing plugs, I doubt it.