Whilst no-one can deny Amazon's current dominance in the cloud computing space, this position can be easily undermined by the development of an ecosystem around alternative standards.
Enter GoGrid's announcement of the open sourcing of their Cloud Center API.
Should the GoGrid API gain support and multiple providers adopt it, then Amazon could find itself on the outside of a growing ecosystem and its users facing an uncomfortable prospect of future migration to a standard.
In a straight battle between a centrally planned environment (aka an attempted monopoly) and a marketplace, the market will win. Furthermore, most businesses have enough experience of lock-in with proprietary technology to know it's best avoided.
At the infrastructure layer of the computing stack (where both GoGrid and EC2 reside), Eucalyptus is the closest thing the cloud has to an open source reference model. Whilst it currently tracks the EC2 API, this could rapidly change in the future.
The question must be whether Amazon will respond with an open source API approach or allow GoGrid to continue to gain mind-share. Whilst they might be tempted by a wait and see tactic, by the time that this open API becomes a clear threat the momentum will be unstoppable.