Today marks the end of the upgrade era.
The differences between Office XP and whatever you are using now are mostly cosmetic and in improved ease of use. Yes, these changes matter, but it's not like XP makes Office tremendously more useable than it is already. Changes do add up over time. Still, Microsoft can't count on the upgrade revenue it once enjoyed yet still needs to invest to support .Net. This project, along with Web services created by other companies, are an attempt to fundamentally change how people and computers interact.