The rumors originated here.
Really, I can see someone saying, "Wow! A 30-homer guy for a 5.00 ERA pitcher? Sweet!" And that person would then resume not thinking about baseball until April. If Cantu's defense was even close to acceptable, I'd be in favor of the trade. Sanchez is promising, but he's still a young pitcher. The "once he finds his control, he'll be awesome"-gag has left many broken hearts, and every fan overrates his/her own young players.
But Cantu is a butcher in the field. You can live with a low-OBP if it comes with substantial power, but not if the combination also brings abysmal defense. Cantu is Pedro Feliz, plus ten home runs and twenty points of OBP, minus a glove. And I mean that literally: a bare-handed Pedro Feliz would probably be preferable on defense.
Pass. Hopefully, the Giants are just stringing the Marlins along, trying to pull the ol' switcheroo with Dan Uggla.