In the USA, the main providers of clustered/load-balanced hosting (i.e. "cloud" or scalable platforms) appear to be Cartika, MediaTemple and Mosso. They all have their advantages and disadvantages, so it's worth researching each for their own merits.
If you are looking for scalable cluster services then Google App Engine is the best.
Google App Engine supports a very narrow range of technologies. If the application you want to deploy is supported on App Engine though, then it's a good solution.