Email and internet are increasingly available all over the plateau. Lhasa, Shigatse, and Gyantse have internet, as do Xining, Labrang, Yushu, Dege, Gardze, Drango, Daofu, Xinduqiao, Litang, and Dartsedo in Eastern Tibet. Small towns that receive a high volume of tourists often have internet as well, as do most county seats and increasingly even some township seats. Internet bars charge about 5 RMB per hour of use. The Chinese word for Internet - "Interwang" - is probably the easiest term to use in searching for a connection. Typically internet cafes are really more about gaming than about the Internet, and you will find most seats taken up by kids (and young adults) playing often violent computer games.
Expect that all of your activity is monitored, not only in the T.A.R, but in mainland Chinese cities as well. Even Skype can be listened to and read by the government if one of the participants is inside of China. Popular websites like BBC news are regularly blocked, as are many sites with user contributed content like Wordpress, Blogger, Wikipedia, and YouTube. Outside of the T.A.R. in mainland Chinese cities, many foreign travelers have found that the restrictions on news and media content are somewhat less strict as authorities are less apt to raise an alarm and disable your connection if your happen to read an article about Tibet or Dalai Lama from a mainstream news source.
Expect that your email will usually be accessible, especially if it is a popular service like Yahoo, Hotmail, and so on. Voice-over-internet and video chat typically work as well, though often Internet bars only have support for popular Chinese programs like QQ. If you have a personal computer, some high-end hotels will provide an internet connection in your room (in Dartsedo and Lhasa, for example). However, be forewarned that it can be very difficult to connect your laptop to a connection in Internet cafes unless you are very technically proficient. It sometimes works, but more often will not. In addition, if asked cafe owners will often refuse to let you try, though if you just do it without asking they are usually slow to object.
Wireless hot-spots are rare in Tibet, though are relatively easy to find in large cities like Chengdu at restaurants and coffee houses.
On a more technical level, note that China has a set of restricted terms that will cause you to be unable to access information from a website if one of those terms appears in the URL (web address) of the web page your are using. For example, if you search the term "human rights" on Google, mechanisms in place will recognize the search request and temporarily block the web page that made the request. Other sites will continue to work but Google won't. (In reality, it is a little more nuanced than this: the specific server you are connected to will get blocked. For domains with many servers like Google etc., your next attempt to access Google might be successful because you ended up connecting to a different server than the one you accessed previously). This can be helpful to know if you are trying to access politically innocuous content but included a word in your search that the system interpreted as political. You have not done anything wrong; the system simply was not smart enough to understand your query.
It is possible to use your home cell phone plan from outside of China to consult email and the internet while in China. The best deal, at least for US cell phone users, by far is to use a Blackberry, since they typically offer unlimited email (which does NOT include access to the Web in general). You just inform your service provider, and for a specific fee they will then activate your international coverage and you can use email as much as you like on your phone. TMobile is great in this regards, but you will have to talk with other carriers to see what their policy is. Remember that the rest of the world, and especially China, mostly use GSM, so you have to make sure your cell phone is a GSM phone. In addition, see if you can turn the "international coverage" off and on for specific lengths of time, or whether the company unreasonably makes you pay for an entire year of coverage even though you may only need it for a single three week trip.
At least for TMobile, China works great for emailing with Blackberries, as does India. However, at least as of 2008, TMobile's parter in Nepal does not support access to email through a Blackberry running TMobile.
David Germano, Zach Rowinski