I wonder if this showed up just before final grades were due. Sorry to be jaded, but it sounds a little over-the-top to me. (maybe I'm just jealous)
Not to toot my own horn, but I've gotten a few (and I mean few) letters like that. I wish I could say I got them from every student, but I've maybe gotten 3 in my 7 years as a teacher. My favorite was from a student who is now at VMI. She gave me a Christmas Ornament that she had made with a letter thanking me for teaching her for two years, and giving up so much of my personal time to earn the credits so that I could teach advanced classes that she could take. Not that I need an "attaboy" as a teacher, but it's certainly heart warming when a student tells you how much you impact them. -Matt