How does one ensure Bitcoins?

At the rate agreed upon by the purchaser and insurance provider. The considerations are different than those of insuring gold or dollars, but to an actuary, they're all just probabilities.

I think the question was more, with whom does one ensure Bitcoins? Has any insurance provider stepped up and said that they have a solid-enough picture of Bitcoin's risks to know what premium to charge?

Many of the big web-wallet providers, exchanges, and other custodians insure their customers' Bitcoin holdings against theft. Their insurers include Aon (who insure Coinbase), Lloyd's (Elliptic), Marsh (Circle), and XL Catlin (BitGo).

No, really my question was how can an insurer underwrite bitcoin holdings reasonably without being supremely unprofitable?

What standards of bitcoin security will the underwriter be evaluating against?

Thanks for the info.

Coinbase delays by like... a week though. So a minor delay on price data is still better than the status-quo. We need more BTC exchanges to open up and compete against each other.

That's not really true for Coinbase Exchange, which is real-time. You don't need to wait for a week, just buy/sell at the spot price. No pricing delays, no "market maker prices", which was rightly pointed out by the OP to be front-running and no other gimmicks. Also, no Singapore arbitration.