Although OS X, starting with 10.5, includes support for Unicode Tibetan, it turns out that a number of important Tibetan language sites have not yet reached this point and still use custom fonts with legacy encodings. Examples are Radio Free Asia, SaveTibet.org and Tibet.net. To view these sites you will need to download and install the special font they use, TCRC YoutsoWeb. You might also need to try a different browser, like FireFox or Opera, rather than Safari, and fiddle with its font preferences.
You can download tcrcyweb.ttf here or from my iDisk.
As for official Chinese sites, these seem to be only in Chinese or to use graphics instead of text. An example of the latter is China Tibet News An example of a Chinese site in Unicode Tibetan is Tibet Information Technology Web.