Choosing a name for your domain is an important decision which will have many technical repercussions on the topology of your network infrastructure. Still, choosing a domain name is largely a business decision that is influenced greatly by the organization structure and needs.
The problem is that there is almost nothing more variable than the business environment and its needs. For example an organization restructuring may mean renaming some sub domains and/or changing their hierarchy, while a change of the company name will defiantly force a domain name change (a frightening prospect for many Windows administrators).
I was put in such a situation where a company that I am working with as an IT consultant had grown from a small design company of 20 employees with the name of Untilled Studios to became a successful group of companies employing 150+ person with the name of Think Arabia. For a while I ignored the problem but their needs have also developed and they started to need an internal SharePoint site with the name of the new organization and sub-sites for the different companies. It became obvious that a domain rename was needed.