I know they do rebates all the time, but that doesn't necessarily mean this one has absolutely nothing to do with the 5DM3 announcement, does it? I'm surprised at how quickly/thoroughly everyone is ruling out the possibility. It seems like a fairly straight forward, non-controversial assumption to me. Companies do it all the time at the end of product cycles. (Unless this sale follows the pattern exactly (i.e. a month extension, double rebates w/body purchases). If so, I stand corrected.)
Let's see... Mar-Apr 2010, lens rebate doubled w/ HD dSLRs. Oct 2010 - Jan 2011, lens rebate doubled w/ HD dSLRs. I think that one got extended. Then, one month later - Feb-Mar 2011, lens rebate doubled w/ HD dSLRs. None of those were followed by a 5DIII.
It's not so much that the idea is being dismissed. Yes, if they announce a 5DIII soon, the rebate will have helped them clear stock in advance of that. But the point is that this round of rebates is not intended to specifically clear stock in advance of an announcement. It's a similar rebate to those offered previously, on an approximately similar timeline. Business as usual.