Disclaimer: This is hypotethical, please do not take it seriously.
Ok so there is absolutely no information on the PC version save for little tid bits of info from gaming mags, which are console centric (Game Informer, Play etc.) and thus really shouldn't be counted on for PC info. Since they, like Ubi could care less.
NOTE: This section best read while listening to the Rogue Spear theme music (http://billbrownmusic.com/musicG1.htm)
I have a theory on the PC version, that differs from the current bleak theory. My theory is that Rainbow-Six 4 for the PC is in the early stages of development at RSE (or some Ubi studio), and has no resemblance (save for some graphical resemblances) to the console versions. That it will be a R6 game like any other. It will have a planning phase, a full armory of weapons/ attachments and gadgets. It will have all the characters we know (including the unnamed briefing lady), it will have the 4 fire teams we expect and it will finally have good AI.
Why would Ubi want to keep something this good secret? Who knows, but E3 my hope is they'll announce a new Rainbow-Six title for the PC. That shocases everything that Rainbow-Six has come to be, and that it will have a proper Rainbow-Six name.
Rainbow Six: Renegade Serpent, coming this winter to PCs everywhere. (well ok not that corny, but you get the idea "RS: RS". The classic naming convention).
Sound crazy? Well a man can dream can't he?