Obviously the vast majority of us only use Grasshopper on Windows, but now that the Mac Rhino WIP has Grasshopper integrated, is it safe to assume that one can open the same Grasshopper script on Rhino Win and Rhino Mac in the near future if not already?
That depends. Grasshopper for Mac will run all standard components and a lot of plug-ins, but there are some plug-ins that either do not work, or it is not yet known whether they work on Mac.