Hi Carolle...I've never been to Tibet, but have been to China several times. Foreign tourists are allowed to visit Tibet, particularly Lhasa, but the rules are very restrictive and you might find yourself being followed if you choose to stray off the beaten path too much. Unfortunately, the political situation in Tibet is not conducive to a relaxed tourism industry there...and nobody really knows when this will change. The Chinese government has put down pretty tight clamps on the Tibetan people, and has severely restricted and compromised religious activities, which is why the Dalai Lama left Tibet for Dharamsala in northern India.
With all this being said, there is no reason why you shouldn't try and visit Tibet, it's a beautiful country and could provide you with a chance to encounter a fascinating culture. The Dalai Lama and other Tibetans have been outspoken in their promotion of foreign tourists visiting the area. The idea is that if more foreigners come to Tibet, more people will see the situation there and become encouraged to promote the Tibetan cause.
So, by all means...get thee to Tibet!