On a mobile device using a virtual keyboard, errors in input are more common than with a proper hardware keyboard. For such cases when using password echo mode, it would be better to have a possibility for small delay before input is turned into a masked character.
FWIW, already implemented on meegotouch. On input previously unmasked characters are masked and a timer is started for masking the new input. Masking time stylable, 0 delay to work as Qt currently does.