This is a topic that comes up in a lot of my high level business value pitches. Essentially most medium to large companies have a lot of different systems and applications that are all on different platforms. In many cases they span different technologies as well as different servers. There is a cost to maintaining the servers as well as the applications in addition to end user training costs to train them on multiple different systems.
SharePoint offers a unified platform that can be used to consolidate these platforms to a single unified platform.
In addition there is also a stronger push now to use a single farm to extend to support extranet and public facing Internet scenarios. This is absolutely possible, but there are security, licensing, and scalability requirements that need to be considered.
The big difference between 2007 and 2010 is just greater integration capabilities and the service application model. The service Application Model supports different scenarios such as greater flexibility in deciding which services run on which server, and also connecting services between farms.