Time to pull the plug on Exelon's bid for NRG?

Will merger talks between energy producers Exelon (EXC) and NRG (NRG) ever produce more light than heat? Exelon, the biggest utility company in the country by market value, raised its bid for NRG last week, only to have it rebuffed today as "a step in the right direction," but still not enough to get the deal done.

Exelon increased it offer to $6.9 billion last week. The all-stock deal had been worth some $6.2 billion when it was first proposed last October, a 37 percent premium for NRG shareholders. But since then, Exelon shares have fallen while NRG has seen its stock rise, making the offer less appetizing.