I think the threat level is quite low despite then spew of threads on this subject in the past few days. Most merchants gain nothing from this, miners gain nothing and general users gain nothing. That is most of the network. Even if this was somehow forced on the default client, forks (clients not blockchains) would quickly emerge. In the end, if anything substantial is done on the legal size, Bitcoin will just leave the USA behind. People who can in the USA would switch to an alt-coin and nothing would be gained for the people who want this.
If I was forced legally to do coin validation I would probably just close my bitcoin shop. If all US exchanges were forced to do this, I would barely effect me, I would just spend all of my BTC as BTC. If Bitpay required it, I would switch providers or roll my own acceptance server.
The market does not want this, and the market will reject it. If it is forced, the market will find a way around it.
US dollars will be RFID tracked before Bitcoin operates under some forced blacklist.