Breadcrumbs: where you are or where you've been?

I had an interesting discussion with a colleague about breadcrumbs recently. He was doing some work on design patterns and the issue of breadcrumbs came up. The initial pattern proposal basically mirrored Jakob Nielson's recent article on breadcrumbs. In other words, breadcrumbs should show where a user is in the navigation rather than showing them where they've been. As Nielson puts it, "Breadcrumbs should show the site hierarchy, not the user's history."

Of course, the obvious problem with this is that many sites are not hierarchical. So Nielson immediately follows up the statement above by saying, "For non-hierarchical sites, breadcrumbs are useful only if you can find a way to show the current page's relation to more abstract or general concepts." Further proof that confidence is often more persuasive than knowledge, I suppose.

The interesting question from my point of view was whether Nielson's pithy, unambiguous guidance applied to web applications. In most web applications, users don't navigate directly to sub-pages in the application (one of the primary benefits to hierarchical breadcrumbs, according to Nielson) , applications are often not hierarchical (for example, they include tasks), they discourage the use of the browser Back button (which Nielson think removes the need for history), and there are often omnipresent navigation panes that assist users in understanding where there are in the object hierarchy.

For example, let's say I'm in the process of performing a multi-step process like checking out of an online store. As part of this process, the application provides the option to leave the main task path to perform other actions (like creating a profile). How would a hierarchical breadcrumb trail work in this case? Not at all, as far as I can tell. If the user chooses to leave the profile creation task and return to the checkout process, should they use the Back button? Perhaps, but provided a breadcrumb trail that allows the application to handle the move has obvious benefits.

Perhaps breadcrumb patterns are more complicated than they first appear.