[duskwuff]

There is no contract, verbal or otherwise, because there was no consideration -- Martin would not have received anything in exchange for giving his presentation. (Intangible benefits like "exposure" don't count as consideration for the purposes of contract law.) The conference never had any obligations to him, and consequently he has not lost anything by no longer being engaged to speak.

Even if there had been a speaking fee, the conference could have fulfilled their end of the contract by paying him but not having him speak. There is no scenario under which the conference would have been forced to allow him to give a presentation.

[vapapav]

And conversely, if he had backed out last minute for personal reasons, it would have sucked for the conference, but they wouldn't have been able to go after him for damages.