Yeah it took Tyson and Lewis for HBO and Showtime to team up, and they've been promoting boxing a lot longer than UFC and PRIDE has been promoting MMA events. And while people may want to see Fedor/Arlovski or Liddell/Silva or whoever else, it's not going to make 0.001% of the money Tyson-Lewis did, so there's no way the promotions would even consider going for an idea to do a full-out joint pay-per view.
I'll say if it's a rumor that isn't highly talked about, post a source. Most rumors everyone already kinda knows them especially if they're as big as something like a UFC vs. PRIDE show being talked about. I'm sure more than just the thread starter here would have caught wind of that "rumor".