This is not an attack on bitcoin, this is an attack on Mt. Gox being non-compliant with KYC/AML/Registrations requirements. DHS has seized Mt. Gox assets. They are not going to seize wire transfers from Mt. Gox to customer accounts, nor are they going to seize the assets of compliant exchanges. This episode just reiterates that bitcoin exchanges need to follow the letter of the law or the hammer is going to come down. It's a shame, really, since Mt. Gox has the cash to spend a few dollars hiring decent lawyers to ensure compliance.
Not speculating, but do not take this as a given. The feds do not put customers first. They can certainly seize incoming wires to customers, specifically for noncompliance issues, and have. Just ask UseMyWallet or eWalletXpress users. Or Full Tilt Poker players.
+1111 on this. As a long time veteran of internet advantage gambling and poker playing I can say with some certainty that this smells exactly like a similar situation. (Still waiting for my Full Tilt remission...) Do NOT except the DOJ to care about safeguarding your money, do NOT expect that Mt Gox will be able to stay solvent once accounts are frozen, do NOT expect that you will get the full story from Mt Gox before you suffer losses, etc.
I do not expect this to hurt bitcoin as a whole, in fact the point of bitcoin is to be able to route around interference like this. But at this point, if you are holding money in Mt Gox when there are other well-capitalized exchanges (that granted, might also someday face similar attacks from countries, especially the US) and willfully ignoring the writing on the wall of a company that is in over their head (saying nothing about their intentions here, just that they seem to be technically and legally in dubious waters) then any losses you suffer are your fault.
Simply no reason not to shift bitcoins to a different exchange unless you are a daytrader that depends on as much liquidity as possible.