A lot of people still seem to be using 3.4, which is unfortunate. I think our plan (still not fully decided) is to support 3.4 on all available features now, but new features (the tilemap editor, for instance) will leverage features which are 3.5 specific. The way it is set up now, is that it does work in 3.4 as well, but some interface stuff just isn't there.
That is our current train of thought anyway, it could change.