There will definitely be more than one major cloud vendor
The idea that a single vendor will dominate all cloud efforts is just not realistic.
Unlike search or Web browsing, using cloud resources is not, and won't soon be, a drive-by event.
For the foreseeable future, you will have to actively choose to use the cloud. As such, there is little room for total domination. The cloud requires you to opt-in to participate. Google and Microsoft's Internet Explorer are more passive in their usage.
Even with a new breed of tools like RightScale and GoGrid, you are still an active participant. And considering that the cloud is generally discussed as enterprise nirvana, there are some major flaws in the theory that there will be only one giant vendor. The more likely scenario is a few giant meta-clouds that provide the infrastructure first for customers and then as a channel. After all, few companies can afford to build this level of infrastructure, let alone manage it.
But nobody seems to be talking about Power Laws. Nobody's saying that one day a single company may possibly emerge to dominate The Cloud, the way Google came to dominate Search, the way Microsoft came to dominate Software.
Monopoly issues aside, could you imagine such a company? We wouldn't be talking about a multibillion-dollar business like today's Microsoft or Google. We're talking about something that could feasibly dwarf them. We're potentially talking about a multitrillion-dollar company. Possibly the largest company to have ever existed.
No matter what all of us valley wonks and analyst types say, the cloud business is not bigger than Microsoft, IBM, Wal-Mart, etc. Right now the cloud is as much next-generation hosting as it is anything else. There are some large hosting providers, but none come even close to dwarfing the size, scale and reach of the BigCos.
In reality it's still so early that the cloud is an accessory to a IT strategy, not the endgame.