There are some problems here:

- When called e.g. from `write-file' or similar commands,
  `read-file-name' should *not* discard the default file name.  If
  `x-use-old-gtk-file-dialog' is t, the default filename is used.  But
  also with the new (standard) GTK dialog it should be possible to
  supply the default file name as the "Save as" in Firefox[1] also has
  it.

  Note that `read-file-name' is used by several other commands where
  the default file name should be supplied (e.g. saving attachments in
  Gnus: `gnus-mime-save-part'), i.e. the user can simply hit the "Ok"
  button.

- The additional information text ...

| If you don't like this file selector, customize use-file-dialog to
| turn if off, or type C-x C-f to visit files.

  ... doesn't make sense here and it is confusing.  It should be
  displayed only if `read-file-name' is called from `find-file' or
  there should be an optional argument to `read-file-name' to change
  the text.

> In GNU Emacs 22.0.50.1 (i686-pc-linux-gnu, GTK+ Version 2.8.3)
>  of 2006-08-30 on viandante
> X server distributor `The X.Org Foundation', version 11.0.60802000
> configured using `configure '--prefix=/import/xtra/emacs/HEAD'
> '--with-gtk' '--exec-prefix=/import/xtra/emacs/HEAD-i686'
> 'CFLAGS=-DSITELOAD_PURESIZE_EXTRA=200000 -g''
[...]

> Recent input:
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
> <help-echo> <help-echo> <menu-bar> <file> <write-file> 
> M-x r e p o <tab> r <tab> <return>

> Recent messages:
> (emacs -Q /tmp/foo.bar)
> For information about the GNU Project and its goals, type C-h C-p.
> Quit
> Making completion list...
> Loading help-mode...done
> Loading emacsbug...
> Loading regexp-opt...done
> Loading emacsbug...done

Bye, Reiner.

Footnotes: 
[1]  GTK-"Save as" dialog in Firefox: