I saw this on a couple of Mets blogs today. I'll link to them both, as they show different pictures. First, from The 'Ropolitans, and second from Metsblog.com.
Update - 5:04pm:
Check out Mets Merized Online for more pictures of each marker. Of course, they came from StadiumPage.
The Mets have marked the site of Home Plate from Shea Stadium somewhere in the parking lot. I'll be heading to the game tomorrow and I can try to check it out on my own.