I drew this Visio diagram back in 2001 to describe how legacy applications are re-purposed to produce web services that inspired the Pick 'n' Mix Business section of my book. The problem back then was that companies couldn't see the point of doing this. The idea of publishing services that can be consumed by partner applications seemed too far fetched even though this was the stated aim of the new technologies - ambiguously termed Web Services (SOAP, XML, UDDI).
Today the Web 2.0 world is producing services in 'native' form. That is, fully formed standards compliant APIs or widgets as they increasingly being referred to. So when will companies come out to play? Instead of aggregating content or useful search routines, web sites (and blogs) can be built comprised of third party transactional services giving rise to the ability for anyone to become an intermediary serving a specfic niche market.