UTF-8 (8-bit Unicode Transformation Format) is a variable-length character encoding that is used to represent Unicode-encoded text using a stream of bytes. (en:UTF-8, de:UTF-8, fr:UTF-8, ja:UTF-8, sv:UTF-8)
All wikimedia wikis are now using utf-8. There have been some editing problems but the workaround now in place for IE for Macintosh seems to have mitigated most of these. Extending the workaround to other browsers is just a matter of researching what should be on the list, putting together the regexps to match them, and getting a dev to commit the changes).
Which browsers cause problems?