I just discovered Scrivener and really like it! Have a Mac at home and Windows at work and do writing at both places. Any workflow suggestions for going back and forth other than exporting and importing RTFs? For that reason up until this point I have always done my writing using an application that as both Mac and Windows versions.
Working with RTFs is pretty much your best option, though I recommend coyping and pasting the contents of the incoming RTF into the existing document you wish to update in Scrivener. The reason for this is that it retains your existing keywords and other meta-data. Using this workflow, you will notice that inline annotations and footnotes will be lost, as might a few other features such as images and tables, sometimes even lists depending on what you use to edit the RTFs in Windows.
It is not ideal, but honestly there isn’t much better anywhere else. Most of the problems arise around the deficiencies of Mac OS X’s RTF implementation, which Scrivener uses.
Some others who use this workflow might have some better insight and ideas than I have, though. I am lucky enough to have a Mac at work and at home.