Is the Guest account a security risk with full disk encryption on Mountain Lion?

No, it's not. Some say it's a benefit if you have geo-location and remote wipe configured (or other similar software) as it increases the chance that someone finding the Mac will connect it to the Internet.

What's happening is that you also have Find My Mac on. That enables a Safari-only guest account which will allow users to log in and run Safari. The goal is that if someone steals your laptop, they might log into the guest account and give Find My Mac a chance to phone home.

The Safari-only guest account uses the unencrypted recovery partition to launch Safari. It does not decrypt your FileVault area at all, so your data remains safe.

(I've found an Apple knowledge base article confirming this.)