Depends on how/why you met. If you met them in some way unrelated to research/school, then it's not relevant and should not be mentioned. If you met them individually to talk about your graduate research interests, they will likely remember you, in which case it's not necessary.
In a case where you met them at a conference, indicating this in your SOP may be helpful. For example, something like this:
"My undergraduate thesis, supervised by Dr. X, examined the effectiveness of a mindfulness-based intervention for dysphoria in first-year college students. I presented this work as a poster at the 2010 meeting of the Association for Psychological Science. This presentation afforded me the opportunity to disseminate my research, improve my presentation skills, and to discuss my findings with scholars in the field, including Dr. Y. I am currently conducting a follow-up project to examine..."
...would be entirely appropriate, and may help to jog their memory. 🙂