Despite the Giant's insistence that Cain was untouchable, Dave O'Brien for the Atlanta Journal Constitution (in the comment section) says the Braves are losing patience with the Padres on Peavy and are going to use their prospects to acquire Cain instead.  I'm of the opinion that this is just a bluff by the Braves. 

Don’t forget a name often mentioned here, Matt Cain, as another potential Braves target. At GM meetings, the S.F. folks didn’t seem to know either way if the Giants would trade him this winter. If they do, and if something happens with the Braves’ Peavy offer, the Braves could always use some of the pieces they offered S.D. to try to get Cain from S.F.

Even if Cain were available, the Rangers could top any offer the Braves were willing to make. Cain, unlike Peavy doesn't have a no-trade clause.  That no-trade clause artificially lowered the demand so that the Braves can get Peavy without having to give up a guy like Hansen.  That's not going to be the case with Cain and the Giants.  If the Padres are smart, they'll call the Braves' bluff and say they are going to hold on to Peavy.  It's not like they are just dying to move him anyway despite what O'Brien says.  The Braves will come crawling back eventually.