I've always thought that the 360 version of the FFXIII wouldn't steal that much in the way of sales from the PS3 version, but instead they were tapping a market that would otherwise remain untouched. I think for Sony it was important to keep Versus exclusive, as fans of the franchise would logically buy a PS3 instead of a 360 since they would want to be able to play both games in the future. (you would think anyway) So I think a fair bit of the 360 software sales will be from people who wouldn't have had a PS3 anyway. I had 2 friends buy a PS3 last week for this very reason, since they will want to play Versus in the future. I know that is small anecdotal evidence, but I'm sure that kind of sentiment isn't rare.
That's not to say the PS3 is losing no sales, I just don't think the impact is as great as others think.