Tibet visa

Hi, we'll have a China tour in September 2011 (from Italy). We would like to see also Tibet (3-4 days). I read that it's suggested to request visa for Tibet only when we are already in China and that, at that point, it will be released only a group visa but at the end it is not a real group and you can go around alone. Is it true? Where can I ask this Tibet Visa in China at Bejing? And How long does it takes to have it?
I'm a bit scared to book a flight (or a train) to Lhasa and then findout some issue when I'm already in the middle of China trip (staying only few days in each location).
Thanks in advance for any answer.