Where, exactly, is the sweet spot between ultra portability and mega functionality?

In recent months, several hardware developers have been pondering this question, blurting out "Eureka!," and creating a product that's an ultra ultraportable computer, a smart phone that thinks it's a laptop.

Apple's iPhone is perhaps the most critically applauded effort thus far to squeeze something resembling a computer into a handheld device. But by no means is it the only contender. Here are three other recently released or upcoming devices from companies that are trying to redefine portable computing.

Fujitsu's LifeBook U810

Why You Might Want It: Fujitsu claims the LifeBook U810 is the first convertible touch-screen tablet PC in an ultramobile size. When the clamshell-style device is open, its QWERTY keyboard is available. When it's being used as a tablet, a finger or a stylus can be used for input on the 5.6-inch screen. The $999 (starting price) device weighs 1.56 pounds, measures 6.73 by 5.24 by 1.26 inches, and is expected to go on sale in September.

The LifeBook U810's standard four-cell lithium-ion battery promises 5.5 hours of life, according to Fujitsu. The device supports 802.11g wireless networking, has 1GB of memory and a 40GB hard drive, runs standard PC applications, and can be configured with Microsoft Windows Vista Business, Windows Vista Home Premium, or Windows XP Tablet PC Edition 2005.

Why You Might Not: Outside of vertical markets such as health care, tablet PCs have yet to gain popularity. For around $1000, you can buy a reasonably powerful laptop with a bigger screen and keyboard, such as the Asus A8JR-4P021C ($1100), which ranked 80 (Very Good) in our tests.

FlipStart Labs' FlipStart

Why You Might Want It: The FlipStart looks like a miniature laptop. It has a 5.6-inch screen, measures 5.9 by 4.5 by 1.36 inches, weighs 1.5 pounds, has both a track stick and a mini-touchpad for mouse input, and features a jog wheel for navigation.

The device also has a 1.3-megapixel still/video camera above its keyboard, comes with built-in Wi-Fi and Bluetooth connectivity, and can support EvDO mobile data services (you'll need a service contract to use that feature). A 2-inch LCD on the top of the case lets you quickly view your Microsoft Outlook inbox, calendar, and other information.

Why You Might Not: At $2000, the FlipStart is pricey, and the unit that we tested delivered so-so performance. But our reviewer, Dennis O'Reilly, preferred the FlipStart over the OQO Model 02 ($1998), another tiny portable PC. Note: On September 25, FlipStart reduced the price of its device to $1499. --Editor

HTC's Advantage

Why You Might Want It: HTC's Advantage is a hybrid smart phone/ultramobile PC with a QWERTY keyboard that's magnetically attached, so you can remove it when not in use.

The Advantage has a 5-inch touch screen; measures 5.2 by 3.8 by 0.6 inches and weighs 12.7 ounces; and runs the Windows Mobile 6 platform with support for standard Office apps, including Outlook, Word, Excel, and PowerPoint. You can get a data connection using Wi-Fi and UMTS, HSDPA, and GPRS/EDGE cellular networks and make voice calls on a GSM network. The Advantage even includes GPS navigation.

It's available unlocked for $850 from Amazon.com and other online sources.

Why You Might Not: The Advantage will likely seem too big to comfortably use as a mobile phone and too small to easily use as a laptop replacement.

For More Information

• Read our reviews of the Sony's VAIO Micro PC and Samsung's Q1 Ultra-V.
• Check out our recently updated Top 10 Ultraportable Laptops chart.
• Compare the FlipStart to the OQO in our video, "How Small Is a Subminiature Laptop?"

Mobile Computing News, Reviews, & Tips

Video of Motorola's RAZR2: In a video demonstration, PC World Senior Editor Denny Arar shows off the new Motorola RAZR2 and explains how Sprint, AT&T, and Verizon are using the phone's exterior screen to differentiate their own services.

How to Do Dial-Up: Many of you have probably forgotten how to set up a dial-up modem connection. If you find yourself in a spot without wireless or Ethernet connections, read Steve Bass's refresher course.

Nikon's New Wireless Digital Camera: Among the eight digital cameras Nikon recently introduced is the $330 Coolpix S51, which Nikon says makes it easier than its earlier wireless cameras to connect to a wireless network. You can send images and, now, videos wirelessly directly from the camera. The camera has 8.1-megapixel resolution, a 3-inch LCD screen, a 3X Nikkor zoom lens, and other features.

Suggestion Box

Is there a particularly cool mobile computing product or service I've missed? Got a spare story idea in your back pocket? Tell me about it. However, I regret that I'm unable to respond to tech-support questions, due to the volume of e-mail I receive.

Contributing Editor James A. Martin offers tools, tips, and product recommendations to help you make the most of computing on the go. Martin is also author of the Traveler 2.0 blog. Sign up to have the Mobile Computing Newsletter e-mailed to you each week.