The lack of a central issuing party isn't a problem - they can go after any point that converts them to get "real money" or provides goods subject to control.
Physical goods are a subset of the latter.
If you can't buy food or pay rent with Bitcoin, how much can it catch on?
Securities are more closely regulated than commodities in the US. I'm not sure what the laws are in Japan, where Mt. Gox is located, but I expect they could find some legal pretense to raid the exchange. There may be more exchanges popping up all the time, but almost everyone uses Mt. Gox. As a result, they're holding a huge amount of bitcoins and digital cash. That makes it a major target for a raid.
I wonder if they will start keeping backup servers in multiple countries, like Wikileaks? I'm not sure if that would protect peoples' digital cash, but their bitcoins would be safe in the event of a raid on the main server.
Edit: Also, it looks like Mutum Sigillum LLC, the company that processes Mt. Gox's deposits and withdrawals through Dwolla, is incorporated in Delaware. I wonder if they process those transactions in the US?
Is that just conjecture or do you have some evidence?
I hold bitcoin and expect it to prosper in time; I also expect a crackdown if it does succeed, but I would be very surprised to see anything soon.
It's hard for them to touch: the existing currency laws certainly don't apply. The more-likely scenario of them becoming stores of value and/or securities (legally speaking) gives them little room for enforcement as there is no central issuing party to go after.
I think the most likely crackdown will happen at the exchange level; but the only major one is in Japan, with new ones popping up everywhere within 6 months.
Honestly, the market is so small right now I would expect nefarious market manipulation as the main course of action for at least another order-of-magnitude growth in bitcoins market cap.