If in Apache I set a Dav directory with the following limit (read is allowed for everyone, but only authenticated users can write to it):

<Limit POST PUT DELETE>
     Require valid-user
</Limit>

Using a dav:// URL, Nautilus will access this directory in read mode, which is OK. However, if I try to put something into the directory, Nautilus won't prompt me for a password and just give me an access denied error.

If I try the same thing using Konqueror (with a webdav:// URL), Konqueror will also give me normal access in read mode, but if I try to put something into the directory then it will prompt me for a password and the transfer will then successfully occur: that is the expected behavior.