Probably nothing. If it became a problem someone would likely release a patch to the client or standalone utility which would prune the tx from a user's local copy of the blockchain.
Can't. your node has to keep a copy of the block to give to other nodes when they ask for it. every block past that point relies on that hash, and that hash relies on the merkle root, and that merkle root relies on the hash of the transaction data. And that's just the first thing I can think of. BitcoinQt/bitcoind probably can't run with a corrupt block and there could be other problems.
I don't know much about "light" clients, but there could still be ways to make those nodes download that transaction.
Of course I doubt it would even go that far. Most child pornography laws specify what is considered an image. A non-viewable binary representation of an image likely doesn't qualify. I mean if it did then one could for example write a piece of software which takes uses a ebook copy of the Bible (or insert random book here) as input and based on instructions in the software display an image depicting child abuse. Would that make the Bible (or insert random book here) child pornography?
The data would be written in blk00x.dat in plain format. It would be like appending a jpeg to the end of a file and then writing some more stuff on top of that. very easy to extract.
apart from anything else, all they need is an excuse, and they would have it. and don't forget about the negative press that would make it easier.