From what I've read on the site below, there actually can be some physical and psychological withdrawal symptoms after discontinuing weed, particularly for regular or heavy smokers. But they're not permanent, and theyr'e not like the sick withdrawal from a hard narcotic. Two of the common ones reported are apparently "anhedonia," or lack of ability to feel pleasure, which essentially is like depression, and lack of appetite. I always trust what Erowid has to say. They've gathered this info from thousands of people and years of research.
http://www.erowid.org/ (See Plants & Drugs/Cannabis)