I'm planning on hiking the PCT and know a couple people who will be hiking the JMT in the opposite direction. We have talked about the first person to pass through leaving a message for the other person somewhere along the trail. Any suggestions on a good place to leave a message? Message boards, trail registers, etc.

Cheers

First that comes to my mind is a shared resupply point, such as MTR (Muir Trail Ranch) or VVR (Vermillion Valley Resort).

People leave notes on the ground under a rock and that sort of thing all the time, it's a common way for thru-hikers to connect with someone behind them, but effectiveness varies depending on how long it's to stay out there.

Another possibility is one of the bear lockers along the trail: http://climber.org/data/BearBoxes.html

Red's Meadow is a close one to the trail also. I don't remember if they had any kind of bulletin board at Devil's Postpile.