Yes. Constellation Energy Group's board is not allowed to solicit competiting offers, but if one lands on the table in certain circumstances CEG can talk. Here are some relevant parts of the agreement, all cited in the proxy statement filed with the SEC:

Notwithstanding these restrictions, prior to Constellation Energy’s shareholders approving the merger, Constellation Energy may engage in discussions or negotiations with, and furnish information with respect to itself to, a person making a takeover proposal if:

-- Constellation Energy’s board of directors determines in good faith, (i) after receiving the advice of its financial advisors and outside legal counsel, that such takeover proposal constitutes, or is reasonably likely to result in, a “superior proposal” of the type described below and (ii) after receiving the advice of its outside legal counsel, that the failure to take such action would be reasonably likely to result in a breach of the directors’ duties to Constellation Energy and its shareholders under applicable laws.

-- Constellation Energy advises MidAmerican of its decision to take such action.

At first glance the proposal by Electricite de France seems to fit the definition of "takeover proposal":

A “takeover proposal” means any inquiry, proposal or offer from any person or group of persons other than MidAmerican, Merger Sub or their affiliates relating to:

• any direct or indirect acquisition or purchase of a business or division (or more than one of them) that in the aggregate constitutes 15% or more of the net revenues, net income or assets of Constellation Energy and its subsidiaries, taken as a whole, or 15% or more of the equity interest in Constellation Energy (by vote or value).

Constellation's board is contractually obligated to recommend shareholders vote "yes" to Buffett's proposal on Dec. 23. But here are the terms under which it can wiggle out. If Buffett doesnt' make a counteroffer, Constellation is still obligated to submit his original offer to a shareholder vote.

Constellation Energy agreed that it will not, and will not permit its subsidiaries or joint ventures or its and their officers, directors, agents or representatives to, withdraw, modify, or amend the recommendation of Constellation Energy’s board of directors in a manner adverse to MidAmerican or Merger Sub unless prior to the approval of the merger by Constellation Energy’s shareholders:

• Constellation Energy’s board of directors determines in good faith, after receiving the advice of its outside legal counsel, that the failure to take such action would be reasonably likely to result in a breach of the directors’ duties to Constellation Energy and its shareholders under applicable laws;

• Constellation Energy advises MidAmerican of its decision to take such action and, if the decision relates to a takeover proposal, the material terms and conditions of the takeover proposal, including the identity of the person making such takeover proposal;

• Constellation Energy gives MidAmerican five business days after delivery of each such notice to propose revisions of the terms of the merger agreement (or make another proposal) and shall have negotiated in good faith with MidAmerican with respect to such proposed revisions or other proposal, if any; and

• Constellation Energy’s board of directors shall have determined in good faith, after considering the results of such negotiations and giving effect to the proposals made by MidAmerican, if any, and after receiving the advice of outside legal counsel that the failure to effect such withdrawal, modification or amendment of the recommendation of Constellation Energy’s board of directors would be reasonably likely to result in a breach of the directors’ duties to Constellation Energy and its shareholders under applicable laws.

Notwithstanding a withdrawal or modification of the recommendation of Constellation Energy’s board of directors, Constellation Energy is required to submit the approval of the merger to the vote of its shareholders unless the merger agreement is otherwise terminated. See “The Merger Agreement—Termination” beginning on page 84 for a discussion of each party’s ability to terminate the merger agreement.