From the Star-K's article on Tevillat Kelim:
Utensils require tevila with a brocha when they have direct contact with food during preparation or meal time and are made from metal such as aluminum, brass, copper, gold, iron, lead, silver, steel, tin, or glass such as pyrex, duralex, and corelle.
Isn't a meat thermometer metal, and doesn't it have "direct contact with food during preparation"?
The article also mentions:
Utensils used exclusively with raw, non-edible food, for instance cookie cutters or a metal tenderizer hammer do not need tevila.
One could argue that this would apply to a meat thermometer as well, but is a meat thermometer only used with raw, non-edible food? Isn't it also used to measure when the meat is done cooking, which would mean that the meat was in an edible state?