BTCguild with their 1 hour delay on stats makes the simplest PP system practically hopper proof.
I doubt that, hoppers are just too lazy to hop it properly.
It's true. You can still hop it in exactly the same manner as before, it just is less effective. To make proportional hopper proof you need a delay that is longer than a round could ever conceivably be.
Actually, I was thinking about hopping without using the pool's published stats at all. When a block is found on the network which is not by any of the transparent pools, assign a certain probability to it in being from each of the delaying pools based on their hashrate. Calculate expected efficiency for such pools based on these probabilities. When efficiency is higher than other pools mine there (this will usually be when several hidden blocks are found in a relatively short time).
And that's without using technical methods to figure out block origin, of which I know very little (something to do with network traffic, long polling, etc.)