The following multilingual site (English, French, Japanese) runs utf8_unicode_ci:
http://nashi.podzone.org/
As I understand it, the collation tells mysql which characters to treat as being the same when doing string comparisons. With the utf8_unicode_ci collation each unicode character is treated as completely independent. So two strings will only match if they are exactly the same. A database operation which involves string comparisons like the following:
where the first string is written using Japanese full width alphanumeric characters and the string on the right is written using plain latin characters, will return FALSE. If you are using utf8_general_ci however, then the two strings might be considered equal and the query might return TRUE.
Hope that helps.
(Shamless plug: If you are thinking of developing a multilingual website, you might want to consider using
YAMS).