Brandon replied to the follow-on question, so I'll reply to the original question:
 
The un-rendered display of Velocity references that evaluate to null (which is what this is) can be suppressed by adding "!" to the reference, right after the "$" - e.g.: $reference or ${reference} becomes $!reference or $!{reference}.  That will stop the obnoxious display of data meaningless to the user, but I suspect that's not what you want.  Instead, I think you want users to see the same thing as they would if logged in.
 
The problem, of course, is what causes the references to be null in the first place.  If the information that is supposed to be displayed is not intended to be hidden from unauthenticated users, then access to the resources accessed by the reference needs to be granted to them (i.e., to XWikiGuest).  In the specific case of ${pdoc.displayTitle}, I believe that the resource is the relevant Preferences page, i.e. XWiki.XWikiPreferences or <spacename>.WebPreferences.  This is something I've seen, and I think that the default permissions on those pages are probably wrong.
 
It may also be due to lack of permission to read the "skins" or the "templates" directory.  The URLs for these docs will be (relative to the appserver's address: /xwiki/skin/default/<docname> and /xwiki/templates/<templatename>.vm.  I hope that's not the case, because I really don't know how to fix that.
 

brain[sic]


 
On 2/8/07, Ipacs Péter <peter@ipacs.hu> wrote:
Hi there!

Is it possible to hide " ${pdoc.displayTitle}" and similar texts from the navigation bar?
It's renders the proper title of the doc if the user is logged in but displays the  "${pdoc.displayTitle }" (and similar) strings if the user is not logged out.

Anybody faced the problem? Solutions?

Cheers,
P