If the government made accepting bitcoins illegal it will be the greatest thing that could have happened to bitcoin. Look what it did for reefer.
The only difference is that pot is a tangible good. What good is a virtual currency if you can't spend it? If no one will accept Bitcoins as payment then they're useless. If the government outlawed Bitcoins it would pretty much be the end of it. I feel like I'm repeating myself.
Bitcoins are made an illegal currency and companies can no longer exchange BTC for USD. Companies no longer accept BTC. BTC prices plummet. How hard is that to understand?
You are assuming that there will not be other sources of demand. The world's black market is huge. Imagine how useful to people in this market an anonymous digital currency would be.