I wonder if Namecoins has just uncover a flaw in the Bitcoin system.
Just take a look at what happen with Namecoins now, someone just brought up some serious hash power, taking the dificulty from 3500 all the way up to 55k, then retired back to Bitcoins (i guess).
Now Namecoins are on a depresion, pleople stoping mining them, next dificulty is stimated at 16k, but at this rate it will not happen until 07/08/2011.
If you want to crush a crypto currency like this and you have lot of hasing power, i think the best plan is to increase the hashing power up gradually, them retire. Imagine what could happen to Bitcoins if the hasing power goes back to 2 thash/s now...
I suspect that because bitcoin was the first, the hardware and software to mine with GPUs was not available. Since when namecoin came online everything was in place, so I see no flaw here. The network is adjusting for the huge increase in mining that happened a few weeks ago with namecoins. So thus, the difficulty adjusting back down is just a result of it not being worth mining it at the current time. I suspect when bitcoin's next difficulty surpasses 1,800,000 and namecoin readjusts downward, you will see a rush into namecoin mining once again.