Of course this was already in the works for quite some time but I still think its an important announcement. Using Office Open XML means a lot for software developers, not only does the XML mean we can now actually write programs that generate Office document a lot easier without using Office itself, always a big no no in a server environment. But it being a standard means we can write programs and expect the to work when the next version of Office is released. Of course the standard also means it is a lot easier to create al sorts of commercial tools so expect a nice and growing third party market for Office add-ons
See: http://www.ecma-international.org/news/PressReleases/PR_TC45_Dec2006.htm for the official press release from Ecma.