Jakob Nielsen writes on Information Foraging: Why Google Makes People Leave Your Site Faster:

Google and always-on connections have changed the most fruitful design strategy to one with three components:

• Support short visits; be a snack
• Encourage users to return; use mechanisms such as newsletters as a reminder
• Emphasize search engine visibility and other ways of increasing frequent visits by addressing users' immediate needs

Better intra-site navigation and better site maps may tip the balance slightly back in favor of longer stays, but it's safest to assume that users' visits to any individual website will become ever shorter.