Version:            (using KDE KDE 3.5.5)
Installed from:    Ubuntu Packages

Use case (what kfile does at the moment):
User is editing a document.
They open the "Save as..." dialog to save it
They type in "notes" as the filename
They then click on "Save".
A dialog appears telling the user that a file with that names exists already. They do not wish to overwrite it so they click cancel.
They pick another filename and save the new file without overwriting.

I suggest a small usability enhancement to give quicker feedback to the user that the filename is already taken. Each time the user modifies the potential filename, kfile should immediately check to see if that file exists already. If it finds a file with that name already, it should be indicated immediately in some passive way in the "Save as..." window.

One suggestion is that the location text entry box changes colour, such as to red as the operation will be destructive. Firefox does something similar with inline search, but when your result cannot be found. Another suggestion is for some red text to appear next to the location bar saying "This filename already exists".

If the location being saved to is a slow remote location (e.g. ftp) this kind of feedback may not be reliable, but would still be a good idea in terms of quicker feedback in general.