In an earlier article I mentioned some fonts are now available for the new scripts in Unicode 5.1. If you try to use these in Pages (or Keynote or iWeb), you may find that they do not work. For some reason the text engine in these apps does not allow direct input of characters not included in the Unicode version embodied in the OS -- i.e. characters which do not have a name when you select them in the Character Palette.
Strangely, this bug only affects characters in the Unicode BMP (Basic Multilingual Plane) -- direct input of those at U+10000 and up is not a problem.
A workaround is to compose in TextEdit (or another app) and copy/paste into Pages.
Another bug is the apparent inability to input ZWJ and ZWNJ in Pages. These characters are required for correct encoding of some languages using the Arabic, Tamil, and Devanagari scripts.