This is a FAQ.
There is no interoperable way to do this. Some browsers implement proprietary extensions (IE, Chrome), other implement RFC 2231 (Firefox, Opera).
See test cases at http://greenbytes.de/tech/tc2231/.
Update: as of November 2012, all current desktop browsers support the encoding defined in RFC 6266 and RFC 5987 (Safari >= 6, IE >= 9, Chrome, Firefox, Opera, Konqueror).