It's important for Google to get its privacy and security policy right with Gmail, because Gmail is the standard-bearer for an increasingly important approach to Web programming called Ajax, for asynchronous JavaScript and XML. Simply put, Ajax applications have user interfaces that run inside a Web browser, but the heavy computation and data storage are done remotely -- in the case of Gmail, on Google's supercomputer cluster. When you start up Gmail, large parts of your in-box are downloaded into your computer's memory and displayed in your browser as needed. This makes Gmail dramatically faster and more efficient than existing Web-based mail systems, where messages and mailbox lists have to be downloaded again and again every time you display a new Web page.

In recent months, Gmail has introduced a message editor that lets users bold and italicize text or change fonts within a message -- much the way you can in a PC-based e-mail program like Microsoft Outlook. There's even an "autosave" feature, so that if your browser crashes you don't lose the message that you were composing. And Gmail can now be integrated with Google Desktop; for example, you can download your e-mails to your Windows-based computer and search and read them when you are not online. All of this is made possible by Gmail's Ajax architecture.

So if Google is applying Ajax with such skill, why am I still concerned about privacy and security?

When most people think about privacy, they think about the threat of accidental disclosure of personal information. When they think about online security, they tend to think about worms, viruses, and phishing attacks -- active attacks by bad people or bad software.

But privacy and security are more complex. Privacy, for instance, includes not just the right to keep personal matters out of the public eye but also the right to be free from intrusion -- the right to be "let alone," as Samuel Warren and Louis Brandeis put it in their famous 1890 Harvard Law Review article "The Right to Privacy." Gmail's advertisements may be less intrusive than those of Hotmail and Yahoo, but they are intrusive nevertheless.

Google argues in its updated privacy policy that users should have the right to choose to read their e-mail through a free, advertiser-supported service. But of course, Google does not in fact offer a choice: there is no fee-based, advertising-free version of Gmail. I note this not to be obnoxious -- clearly, Google can argue for choice in the market without itself having to offer more than one option -- but to call attention to the most important characteristic of Google's business model.

That characteristic is this: fee-based consumer services are not part of Google's business model at all. Although Google is often called a search company or an e-mail provider, it earns its billions by selling clicks on targeted advertisements. Everything else is merely the honey designed to attract enough attention that some of it will spill onto those ads. Gmail's users are not Google's customers; they are its product. I personally find advertisements highly distasteful and have shied away from Gmail for that reason.