Not to go against the grain, but I thought it was a rather obvious penalty on Chancellor, though I'm glad he did it anyway. By the letter of the rule, that was a clean hit, but just as laws are not always set in stone, rules aren't always enforced to the letter, they tend to be enforced on precedent. And recent precedent in the NFL is that any airborne "defenseless" receiver is going to draw a flag if he gets rocked by a vicious hit mid-air, even a clean hit. I hate that the NFL calls it that way, but they've consistently called it that way for several years now. If the refs had let it go, there would have been tons of other NFL players complaining about getting flagged for similar actions in other games.
Don't expect the NFL to change that stance any time soon. Goodell is trying his best to stem off the inevitable massive lawsuit against the NFL from players claiming permanent brain damage. I wish him luck, but that lawsuit is probably coming anyway.
As far as the play, I'm glad Chancellor did it. Obviously, I'm not glad that Vernon Davis was hurt during it. He's a human being. But as far as the play itself, if Chancellor doesn't go all out that play might be a touchdown, or 1st and goal inside the 2 yard line at the least, which is almost the same result. The result of that play/penalty was 1st and goal at the 10 instead of first and goal at the 2. That's a huge difference, and well worth the flag. Hopefully Davis is okay. I dunno though, I think he might have shortened his life a little with that hit. To me I'd rather dislocate my knee like Manningham had than nearly jar my brain loose and know that dementia or extreme depression could be coming for me in twenty or thirty years.