You can use your iPhone’s built-in microphone and Voice Memos app to record lectures. To optimize quality, take a few sensible steps before the event. Learn more in my article, “How to Record Lectures with Your iPhone’s Microphone,” published online at eHow.
Update (June 2012):
You can attach a microphone accessory to your iPhone to use instead of its built-in mic. Two accessories in particular lend themselves well to recording lectures. See my latest post on the subject, “Two App Accessories to Better Record Lectures, Seminars and Presentations with an iPhone or iPad.”