Since this topic comes up every now and again, allow me to point out a resource I just happened upon:
On this page is a link to an XML file that is a very thorough database mapping Unicode code points (at least, ones that we care about) to their LaTeX representation. When the LaTeX representation requires a particular package or environment, these are provided. For example:
<entity name="Kcy" set="iso-8879-cyr1">
<desc>=capital KA, Cyrillic</desc>
<desc>CYRILLIC CAPITAL LETTER KA</desc>
<seq req="cyracc,amsfonts" env="cyr">K</seq>
This is the mapping to end all mappings. It evens comes with convenient PDF references.
"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter