There are a few things I will look for on either side of legitimacy:

• Bitcoin is made more legitimate by a large corporate player becoming vested. This could be in the form of a new exchange such as Tradehill, which now operates on an "institutional level" (you need about $10,000 USD to open an account) or even an established brokerage such as Paypal.  Really what it comes down to is someone willing to pay the legal fees to fight if the Justice Department comes knocking.  It's clear by his comments that Gavin Andresen would put up no fight.  Maybe the Winklevoss twins would with their stake.  I need to see some positive legal precedent to move forward.
• Bitcoin goes through a major legal apocalypse... and survives underground.  I definitely see Bitcoin as a currency and P2P system continuing to exist regardless of any attempts to shut it down.  The nature of it's protocol and the hardcore community will keep going even if it has to resort to being TOR network only.  At that point, once the common methods of the underground existence (new exchanges) of Bitcoin is established I'll evaluate my options.  Case in point: Napster to Emule to Torrents... the idea is unkillable it just morphs.

The key problem for me right now is uncertainty and high risk of frozen assets/total loss keeping capital (BTC or USD) on website exchanges.  It's too much for me as a professional trader to have a significant portion of my capital on the current mediums with all the government threats facing the market.