Have you been having problems getting your Unicode text to display properly in editors and webpages? So have I, and there is very little practical Unicode information online for OS X, despite OS X "fully supporting Unicode." It turns out that although the OS does, in fact, support Unicode fully, very few applications have yet made the grade.
Out of a maze of technicalities, I have managed to understand the following: Cocoa apps are much more likely to support Unicode "fully" than Carbon apps, and your default OS X Unicode keyboard may not be the best choice.
This boils down to what are apparently called the "composed Unicode" (using a single code point to display one character, regardless of complexity) and "decomposed Unicode" (using two code points to display a character with an accent, one for the vowel, one for the accent) forms of Unicode. Cocoa apps will display both, and so will Safari. No other browser I've used so far will, including OmniWeb, which is strange, since it uses the same base as Safari. Carbon apps, and most browsers, will only cope with "composed" Unicode. What did my default OS X Unicode Vietnamese keyboard input? DEcomposed Unicode. I'd been struggling for months to input something that most applications couldn't display.
After all these months of trying, and searching for applications with full Unicode support, I finally found out about the code points, and managed to find a disk image of three composed Unicode keyboards for Vietnamese on this site.
If you have similar problems, look for additional keyboards that will input composed Unicode, also known as Unicode Normalization Form C. You also want Unicode fonts, and there aren't many available yet. Lucida Grande on OS X will display your text, but if you want to use any other font, you need to tell your readers, and either make it available for download on your page, or link to the download. Your language community may have produced its own Unicode fonts, like these excellent Vietnamese ones.
There's some good practical info on using Unicode on the Multilingual Mac page, including a converter from decomposed to composed Unicode, and you probably can't go too far wrong with Cocoa applications. However, if you want people to be able to read your webpage, stick to composed Unicode: UNFC.
From Clytie, learning her new keyboard layout, which is confusingly reversed from her former one, by retyping _all_ her material...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040915233110907