Each module seems to have its own way of displaying the breadcrumbs navigation.

• The Wiki only shows breadcrumbs for child pages as seen here There's nothing wrong with this.
• The Forums add a useless @Forums » @ breadcrumb one level deep as seen "here":http://www.redmine.org/projects/redmine/boards/1 Well, I think it's fairly obvious you're in the forums and there's no breadcrumbs on other pages with the same behavior so it's very inconsistent.
• The Repository only shows breadcrumbs to what's needed as seen "here":http://www.redmine.org/projects/redmine/repository/browse/trunk/public There's nothing wrong with this.

Having inconsistent breadcrumbs confuses users, and it doesn't help when only a small number of pages actually have them.
I think it would be better if the breadcrumbs were displayed at the very top of the page.

Maybe something like:

Home » Projects » ProjectName » Module » etc...

The last breadcrumb in the list should be the user's location but it will not be an active link. This is to prevent people clicking links and getting frustrated when the page simply reloads instead of taking them a level up.

People have a fairly good view of where they are in the +project+ because of the styled menu links, but I think this way users will immediately recognize the breadcrumbs and know exactly where they are in regard to the +entire site+ as well.
If people change their menu links in their own customized themes a standard breadcrumb trail becomes even more valuable.