So, there is an economic mechanism already in place to make sure dust transactions are paid for.
So that's not the problem that's being addressed.
EDIT: Well, actually, it's a "tragedy of the commons" situation. Having tons of dust transactions is seen as bad for Bitcoin (for reasons that are not yet 100% clear to me). So the devs think there is a need to dis-incentivize that. But the economic mechanism I explained above has not yet kicked in, so right now, you can make tons of dust transactions without having to directly pay for it. The burden of storing those small outputs for all time then falls on everybody who uses bitcoin.
Per the lead developer:
"If you have a better suggestion for fixing the problem of new users wasting lots of time gathering tiny drips and drabs of bitcoins, and then getting upset when they can't spend them (because it costs more in fees that they are worth), I'm open to suggestions."
https://bitcointalk.org/index.php?topic=191425.msg1985498#ms...
If a user is willing to spend $10 to insert a URL in the block chain, there is no way to stop them without making bitcoin useless.
The move to treat very small amounts of bitcoin as simply invalid is intended to reduce the bloat of the unspent address cache (called the UTXO). This is explained at the top of the linked page.
Regarding the CP URL, it was actually several hundreds of links in a large FAQ, and I don't think this changes anything regarding encoding data in the block chain.
(Also, IIRC, wasn't their a proposal to make the data a hash or something mandatory so that it couldn't be random|misc-manual data? Or was that unfeasible for some reason?)