No.  This is an excellent question to illustrate the difference between lobbying and electioneering.  A 501(c)(3) charity can lose its exemption for participating in an election campaign by supporting or opposing a candidate for election for a public office at any level of government.  If Al Gore were a candidate for election, his presentation at your dinner might be considered electioneering.  (See Ready Reference Page: “IRS Issues New Guidance on Electioneering.”)

But since he is not a candidate for office, his presentation on cap and trade legislation, even if imputed to your organization, is at most lobbying.  A public charity is permitted to lobby on legislation so long as it is not a substantial part of its activities.  (See Ready Reference Page:  “Lobbying Rules Create Opportunity for Charities.”)  You might distance yourself from lobbying if you explicitly say that you take no position on any specific legislation.   But even if you take a position at the event—assuming you do much of anything else during the year—that action will not constitute a substantial part of your activities.