UTF-8 chars fails

I think the problem is in the line

$username = preg_replace('/[^a-zA-Z0-9@ ]/', ' ', $username);

in user_import.module, which replaces any thing that is not a latin-1 letters or numbers (including all accentued characters) with a space. I solved this problem by replacing this line with

mb_ereg_replace ( '/[^\pL\pN@ ]/' , ' ' , $username );

which does the samething, except that it accepts the UTF-8 (upper and lower case) letters and numbers not only the Latin-1 letters and numbers.

Let me know if this solves the problem in your case too.