I have no legal expertise, but it sure seems to me that there are a lot of ways to argue for the legality of bitcoin. Even if it were to be outlawed, what would that do? Downloading music is illegal, look how swimmingly that has worked out.
Nobody's actually argued that Bitcoin per se is illegal. The grey areas it falls into at the moment are the ways in which it's used - buying/selling drugs or gambling online with Bitcoin isn't going to make those activities more legal - and the services which have grown up around it (the exchanges and payment processors currently being unlicensed and unregulated).
In the short-term, there probably don't need to be any special Bitcoin specific laws to address those issues. In a lot of jurisdictions, supply and possession of certain drugs are illegal whether or not money changes hands so whether Bitcoin is "money" would only be relevant in terms of trying to seize "proceeds of crime" from dealers (and a lot of places don't require that proceeds of crime be only money, either - pretty much any "asset" can be seized).
Likewise, trying to avoid the laws regarding accepting funds from those in the US for online gambling is an offence in itself according to the US DoJ - that's what the whole shit-storm targeting payment processors and seizing the domains of offshore gambling operators earlier this year was about. Once again, it's not necessary to define Bitcoin as "money" in order to crack down on it being used for online gambling - although it may be considerably more difficult for the DoJ to force operators to return player balances if there's no record of the true owners of those balances.
The continuing e-gold drama suggests that even if Bitcoin became a preferred method of money laundering, it's more likely that the exchanges would be restrained from allowing users to trade and that the government would seek forfeiture of the Bitcoins and funds they hold on behalf of users than that there'd be a need to specifically outlaw Bitcoin.
Maybe, just
maybe, if Bitcoin became a conduit for financing terrorism the government might go after Bitcoin itself. Once again, though, financing terrorism is illegal in and of itself in most Western countries. The Bitcoin economy might also be too small at the moment for it to be useful for moving large amounts of money undetected - significant deviations from established patterns would probably be noticed fairly quickly if counter-terrorism analysts had reason to suspect that Bitcoins were making their way to groups high on the government's watch-list.