How safe is my USD deposit at Bitstamp? With Mt.Gox probably going insolvent/broke in these days, how safe is my USD account balance with Bitstamp? Is Bitstamp as a company registered with some financial institutions? Are customers USD deposits insured? I only want to consider security from the financial point of view and not my account security (strong passwords, two-factor authentication etc. - is a personal responsibility IMHO).

I guess I would lose all my USD balance if the company goes broke?
Bitstamp and any other exchange are companies and so can be insolvent. If you transfer $1000 to them in exchange for bitcoins. Until they send you the bitcoins they are in to debt to you for that amount. You could take them to court to recover that if they failed to provide the bitcoins or a refund. The court could issue an order allowing the companies assets to be seized so you cod get your $1000 back.

However if they become insolvent as appears to be the the case with Mt Gox then there's little chance of you being able to collect on the debt. As there will be no assests left for the court to seize.

Some regulated professions are required to use nominated accounts were money is held on behalf of a customer. In this case the money is still yours. This is something regulation of the BitCoin exchanges might require.