Is there anywhere an exhaustive list of Japanese characters (especially Shift_JIS characters) which are not supported in Unicode ?
Are there any characters in Shift-JIS that are unrepresentable in Unicode? Are any of these non-vendor-specific? (In this case, you'll need a vendor-specific mapping table anyway).
I'm not sure about JIS X 0213 and compatibility ideographs.
You might take a look at http://www.opengroup.or.jp/jvc/cde/ucs-conv-e.html, even though it's probabaly outdated. Another interesting page might be http://www.debian.or.jp/~kubota/unicode-symbols.html; this guy generally seems to be a bit anti-Unicode.
There are some round-trip mapping problems with a small number of characters due to ambiguities (http://support.microsoft.com/default.aspx?scid=kb;en-us;Q170559), but in this case the problem is the conversion FROM unicode, meaning that you can convert character X to character U(X) and back to J(U(X)), but you cannot guarantee that J(U(X)) = X in a small number of cases. AFAIK there are no "unsupported characters" on the back conversion, you might just end up at a different codepoint.
Philipp
