You can probably use Wikipedia to determine the exact implications of the gesture. From the information:
In Afghanistan, Iran and parts of Italy, the gesture is regarded as an obscene insult equivalent to what the middle finger signifies in most of Europe, especially when combined with a sweep of the arms. In these places, it roughly means "Sit on my phallus" and carries the same stigma as does raising the middle finger in the west. The sign has a similarly pejorative meaning in parts of West Africa, South America, Iran and Sardinia, according to Roger E. Axtell's book Gestures: The Do's and Taboos of Body Language Around the World." In southern Sardinia, where this gesture is noted for being particularly obscene, a hitchhiker would be well advised not to wave his thumb in the air expecting a car to pick him up, or he may find himself under the wheels of a large truck.
Though it states that it may be offensive in Israel I haven't found this to be the case.
As per the latest edit to the quoted Wiki page:
The sign has a pejorative meaning in parts of West Africa, Iran, and Greece according to Roger E. Axtell's book Gestures: The Do's and Taboos of Body Language Around the World. In practice, most Greeks nowadays recognize the positive connotation of the sign due to international exposure. Any perceived pejorative meaning of the past is lost in contemporary use. Despite that, the sign is still very rarely used by native Greeks for its positive meaning (and never for the pejorative).
The information may still be subject to change at a later date.