Here's an example of sketchnotes captured after the fact from TED videos by Cristiano Siri. This particular TED presentation, The Power of Time Off by Stefan Sagmeister is a great one to watch. Check out Cristiano's Flickr stream for more sketchnote examples.
Remember, you don't always need to be physically at an event to capture ideas. All you need is a recorded video or even audio from a podcast.
Give it a try — working from recorded video and audio is a great way to practice your skills.
Nice work Cristiano!