Apple envisioned its iPad as a device that defines a new market, something between a laptop computer and a smartphone. Having sold over a million of them in just 28 days, it's clear that the company has tapped into need that other computer makers have been unable to adequately address.

But there's considerable overlap between portable computers, the iPad and mobile phones. At the Web 2.0 Expo in San Francisco on Tuesday, Raven Zachary, president of mobile developer Small Society, explored that territory as he recounted his experience using his iPad as replacement for a laptop.

His was a story not just of Apple's touchscreen tablet but of the cloud, for the iPad wouldn't be as compelling were it not backed by cloud services.

"For me, the iPad has been the cause for embracing cloud computing," Zachary told the audience, about a third of whom said they had bought an iPad.

Whether an iPad makes a suitable substitute for a laptop depends on one's computing needs, Zachary conceded. Developers and designers, people who use Xcode and Photoshop, he said, would have to stick to laptops for the foreseeable future.

But Zachary, who has carried a laptop with him on every trip he has taken since 1998, said he had taken five trips in the past 30 days with only an iPad. His experiment, he said, "has succeed but it has required some work and some focus."

The key to that success, ironically, has been Google Docs and a file sharing service called Dropbox. "This combo has saved me and made it possible for the iPad to be my travel device," he said.

The benefits have included: a lighter burden -- 4 pounds for an iPad bag, compared to 17 for a laptop and accessories in a backpack; a battery that lasts all day; and little things, like being able to work in a cafe and then take a bathroom break without worrying about finding someone to watch gear left behind.

"It takes me about five seconds to pack up my stuff, go to the bathroom and come back," said Zachary.

While Zachary has carried a wireless keyboard with him, he finds that he can type almost as well using the touchscreen in landscape mode.

Nevertheless, the iPad is not without its issues.

E-mail on the iPad needs some work, Zachary suggested, noting that the iPad e-mail app isn't well suited for people like him who get several hundred messages a day.

The problem is that the iPad mail app is ill-suited for filtering and routing mail, features that make large amounts of e-mail more manageable. Despite the increasingly chilly relationship between Apple and Google, Zachary said that Google had done great work supporting the iPad with its iPad-oriented Gmail Web app, which makes high-volume message management easier, thanks to its support for threaded conversations.

Though Zachary was using Apple's Keynote app for the iPad to deliver his presentation, he said that it had some shortcomings, such as poor import fidelity, no way to see projected slides in Keynote simultaneously, and no in-app support for disabling notifications and calendar alerts.

"I don't want to be onstage and have an embarrassing push notification show up," he said, noting that he had to manually disable notifications and operated in airplane mode just to be sure.

Zachary lamented the lack of guest accounts to protect one's files, a feature that would be particularly useful in households where the iPad is shared.

He also observed that "some Web sites just don't work on an iPad." That's not just a Flash issue, he added, noting that draggable JavaScript objects don't work well with the iPad's touch interface.

Finally, Zachary said that iPad is limited in the kinds of documents it can open, printing options, and backup, particularly when away from iTunes.

"The iPad is a laptop replacement for me but not a computer replacement," he said.