That's not the permission of the file you are deleting which matter but the ones from the trash directory. The issue when this directory is not owned by your user is that other users can see things you have deleted which can be a security issue. I've discussed the issue some time ago with Alexander which is gvfs and nautilus upstream and he wrote a mail to the xdg list, you might be interested to read it there, http://lists.freedesktop.org/archives/xdg/2008-February/009192.html
That's not the permission of the file you are deleting which matter but the ones from the trash directory. The issue when this directory is not owned by your user is that other users can see things you have deleted which can be a security issue. I've discussed the issue some time ago with Alexander which is gvfs and nautilus upstream and he wrote a mail to the xdg list, you might be interested to read it there, http:// lists.freedeskt op.org/ archives/ xdg/2008- February/ 009192. html