Here's a breakdown of browsers used to save edits over a day on de.wikipedia.org. I checked de because it's the largest non-English non-UTF-8-using Wikipedia.
Total posts in the day: 2861
Browsers known to work with editing UTF-8: MSIE/Win: 1146 Gecko: 966 [549 Windows; 379 Linux/Unix; 31 Mac] Opera 6+: 331 [291 Windows; 40 Linux] Konqueror: 216 Safari: 137 total: 2796 (98%)
Browsers which can be problematic with UTF-8: old Netscape: 25 ELinks: 20 (not tested, but it's a text-mode browser and these tend to have problems in this area) MSIE/Mac: 10 Lynx: 4 (text-mode again) Opera <6: 2 total: 61 (2%)
There's some margin of error; my regexps lost track of 4 hits or so somewhere in that mess.
I'm actually surprised that the Internet Explorer/Mac quotient is so low, though it's a pleasant surprise. :)
Anyway, adding a notice and/or doing some extra conversion for those 2% shouldn't be a huge server burden.
-- brion vibber (brion @ pobox.com)