That is, hitting RET in the `write-file' dialogue gives you
buffer-file-name, and ignores whatever is in the prompt.  This seems
contrary to what the doc string says:

---
Interactively, prompt for FILENAME.
If you specify just a directory name as FILENAME, that means to write
to a file in that directory.  In this case, the base name of the file
is the same as that of the file visited in the buffer, or the buffer
name sans leading directories, if any, if the buffer is not already
visiting a file.
---

So this isn't an ido problem at all -- it's a bug in `write-file'?  Or
rather...

(let ((default-directory "/tmp/")) (read-file-name "Foo: "))

If you just hit RET there, it'll return `buffer-file-name'.

-- 
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   bloggy blog: http://lars.ingebrigtsen.no