But the most difficult challenge  both to grasp and to solve  of the cloud is its effect on our freedom to innovate. The crucial legacy of the personal computer is that anyone can write code for it and give or sell that code to you  and the vendors of the PC and its operating system have no more to say about it than your phone company does about which answering machine you decide to buy. Microsoft might want you to run Word and Internet Explorer, but those had better be good products or you’ll switch with a few mouse clicks to OpenOffice orFirefox.

Promoting competition is only the tip of the iceberg  there are also the thousands of applications so novel that they don’t yet compete with anything. These tend to be produced by tinkerers and hackers. Instant messaging, peer-to-peer file sharing and the Web itself all exist thanks to people out in left field, often writing for fun rather than money, who are able to tempt the rest of us to try out what they’ve done.

This freedom is at risk in the cloud, where the vendor of a platform has much more control over whether and how to let others write new software. Facebook allows outsiders to add functionality to the site but reserves the right to change that policy at any time, to charge a fee for applications, or to de-emphasize or eliminate apps that court controversy or that they simply don’t like. The iPhone’s outside apps act much more as if they’re in the cloud than on your phone: Apple can decide who gets to write code for your phone and which of those offerings will be allowed to run. The company has used this power in ways that Bill Gates never dreamed of when he was the king of Windows: Apple is reported to have censored e-book apps that contain controversial content, eliminated games with political overtones, and blocked uses for the phone that compete with the company’s products.

The market is churning through these issues. Amazon is offering a generic cloud-computing infrastructure so anyone can set up new software on a new Web site without gatekeeping by the likes of Facebook. Google’s Android platform is being used in a new generation of mobile phones with fewer restrictions on outside code. But the dynamics here are complicated. When we vest our activities and identities in one place in the cloud, it takes a lot of dissatisfaction for us to move. And many software developers who once would have been writing whatever they wanted for PCs are simply developing less adventurous, less subversive, less game-changing code under the watchful eyes of Facebook and Apple.

If the market settles into a handful of gated cloud communities whose proprietors control the availability of new code, the time may come to ensure that their platforms do not discriminate. Such a demand could take many forms, from an outright regulatory requirement to a more subtle set of incentives  tax breaks or liability relief  that nudge companies to maintain the kind of openness that earlier allowed them a level playing field on which they could lure users from competing, mighty incumbents.

We’ve only just begun to measure this problem, even as we fly directly into the cloud. That’s not a reason to turn around. But we must make sure the cloud does not hinder the creation of revolutionary software that, like the Web itself, can seem esoteric at first but utterly necessary later.