we have faced the same problem  mostly i18n parts of
the web  and app  servers are not properly coded.
theoritically you most set the encoding of your html
request form to utf-8 (default is ascii) and this
should solve the problem but most of the web and app
servers I know don't support utf-8 encoded requests
properly and because of this we usually code and
decode unicode characters  in our own code, with java
script in client side and java codes in server  side.

I also had the same problem  and finally
we encoded and decoded ourselves in our codes and scriptles.
it's a bit teasing but most of the time the only solution.

We are implementing this now
the problem  is the posts are working fine with multilingual chars
so its either we go to all requests (post or get) both changed or try
to solve this problem
i m working with ibm on this issue let see how this goes
i will write back on wht r the results

we are using bea weblogic here, putting post into
work is much easier than get since some systems filter
8th bit from http request  and only accept 7bit ascii
urls.
since we have done a lot of work on solving i18n
problems here. we are ready to help you in fixing the
bugs in web  sphere or any other app  server.

the post request  are working fine in my scenario
the problem  is with gets