Found this via diveintomark.org: Usability News - 5.2 2003 -- Breadcrumb Navigation: Further Investigation of Usage. It looks interesting, I need to reread it a couple of times.

In this study, we designed the tasks such that navigational efficiency would be optimized through the use of a breadcrumb trail. Despite this, only 6% of the page clicks were accounted for by the breadcrumb. While 40% of the participants used the breadcrumb trail, usage was lower than that of other navigational means, such as the main navigation bar, the Back button, and embedded links.

Breadcrumb users were found to use the Back button less often than users who did not use the breadcrumb; however, no differences were found in the efficiency measures of total pages visited, navigation bar clicks, embedded link clicks, or time to complete the search tasks.[...]

Location of the breadcrumb trail did have an effect on usage. Breadcrumb trails positioned under the page title (at eye level and closer to other links on the page) were used more than breadcrumb trails positioned at the top of the page. [...] The results also suggest that exposure to a breadcrumb trail in a site may contribute to the type of site model formed by the user. Participants that used a site with a breadcrumb trail were more likely to choose a hierarchical model than those that used the non-breadcrumb site. [...]