Nate Silver puts the spike in context:
The bottom line is that, by the stock market's estimation, the private health care industry appears as though it will benefit if the Senate enacts its plan. But the benefit -- about $16 billion in discounted cashflows -- is small as compared to the total magnitude of the program, and likely reflects an increase in the size of their customer base rather than any anticipation of higher profit margins.
But the deeper point is: why is so much hostility to the bill wrapped up in the horror that private insurance companies might actually make some money off this? That's what private companies are supposed to do. They're constrained from many of their worst and cruelest tactics in this reform, but remain the primary vehicle for it, as was well advertized from the very beginning. It seems to me that many on the left so loathe these companies they'd rather leave people uninsured than allow the insurance industry to benefit. That strikes me as ideology speaking.