I`ve recently read a presentation that mentioned a couple things about how big would be the potential bandwidth if Bitcoins would handle the transaction volume of Visa:
http://www.slideshare.net/dakami/black-ops-of-tcpip-2011-black-hat-usa-2011
It stated, that storing all Visa transactions would take about 1TB of storage per week. This certainly limits the feasibility of using the Bitcoin protocol as an alternative to existing payment processors. Are there any plausible concepts of what part of the economy Bitcoins might be capable of replacing entirely, and which would be harder or impossible to substitute with 'coins?