The Sony Ericsson W580i Walkman cell phone has a lot of cool features but there is one feature in particular that is an awesome innovation. While features like a basic camera, simple video and voice recording, photo sharing and ringtones now come standard in most phones today, what about fitness?

Now that is currently a relatively untapped market for cell phones that has typically been satisfied through other devices or traditional gyms.

So what about embedding fitness functionality in your mobile phone? Now that’s novel – and the Sony Ericsson W580i Walkman has done it with relative ease and effectiveness.

I’m going to focus my analysis on that aspect of the W580i. The rest of what you’d expect in a modern-day phone is there as well (and it comes in black, white, pink and jungle green).

Here is how to find the fitness feature in case you didn’t know about it or couldn’t locate it. Click “Menu” from the main screen on the W580i, scroll down to “Organizer” (it’s the icon with the calculator and the calendar in the middle of the bottom row) and click “Fitness”. Yes, you have now discovered the W580i’s interesting and practical fitness feature. Before you get started, though, make sure to introduce yourself to the W580i now that the W580i has introduced itself to you. In “Fitness” mode, scroll down to “Settings” and then “Personal Data”.

Enter your height, weight, year of birth and gender to improve its calorie-counting accuracy so the W580i can specifically customize itself to you. It is absolutely worth it. On the other hand, you’ll notice “Advanced Calibration” on that screen as well.

I’ve found that important for its distance calibration, but on the flip side, it’s somewhat cumbersome to request of a first-time user. The W580i will ask you to walk a “known distance” and then record how many yards you traveled. Because using yards instead of feet may be a bit boggling for U.S. customers, you’ll need to remember that there are three feet in a yard.

Once you do, things get interesting. First, you’ll notice the W580i has a built-in pedometer. The “steps” feature counts your walking steps based on movement. If you just walk and hold the W580i still in your hand (which I initially did as a test), the steps might not register correctly. In fact, I initially thought the feature didn’t work.

If you correctly slip the W580i in your pocket and walk with a natural sway in your hips, though, it’ll register relatively accurately. So long as you have “step counting” activated, even if you forget about your W580i while it’s in your pocket or bag it will do its job to track your steps.

The walking feature measures your distance (again unfortunately in yards), the number of steps and how many calories you’ve burnt. You’re able to change certain unit settings – i.e. from kCalories to kJoules or miles to kilometers – but not yards to feet. This should be an option.

Along with monitoring your walking, the running function is also based on the phone registering your movement. When you begin running, there’s a bit of a delay for the data measuring to kick in. When it does, it does with relative accuracy. This, of course, is all based on the human accuracy of your “advanced calibration” when you manually measured and input yards based on a known walking distance.

Results are recorded and shown to you in an archival fashion so you can see today’s walking data, information on all your running sessions and information on previous days and weeks. The running feature monitors your distance traveled, time traveled, average speed, maximum speed and calories burnt.

But the statistics monitoring becomes really impressive in the “Advanced” section of your “Results” page. This is a Java-powered application that requires a memory stick and connects to a mobile Web site.

Within this more graphical display, you’re hit with additional training data, eye-friendly graphs on up to 20 sessions, motivational medals to reward you for reaching your goals and fitness trivia.

From an overall fitness perspective, the W580i is genuinely innovative, practical and ultimately useful in support of a healthy lifestyle. But the W580i could do more. The W580i doesn’t measure your heart rate. Heart rate is a critical element for many people who work out and closely monitor their results against their goals. This functionality could have been added with a chest strap that wirelessly communications with the W580i. Take note, Sony Ericsson. This is a missed opportunity

In addition to its dedicated and downloadable fitness applications, Sony Ericsson has been kind enough to include other built-in elements you’d want during your workout. The W580i has iPod-like functionality so you can plug in your earphones and listen to your favorite music tracks courtesy of your phone.

It’s also lightweight (3.3 ounces) and compact (3.9 by 1.85 by 0.5 inches). The W580i’s talk time is rated at a surprisingly high 9 hours with standby time up to 15 days and music play up to 20 hours in music mode.

The embedded SmartChipTM, which signals to your cell phone carrier who you are, is obnoxiously difficult to remove.

After removing the W580i’s battery cover and then removing its battery, I found myself having to use a knife in order to pop out the SmartChipTM. You couldn’t pop it out with your fingers. That’s unnecessarily difficult.

Perhaps the W580i’s “shake control” feature makes up for it, though. You can skip or shuffle music tracks with the mere flick of your wrist.

For a non-smartphone (i.e. a phone reminiscent of a computer’s interface and functionality), this is a premium phone and it’s price as such. The Sony Ericsson W580i Walkman has a retail price of $299.99, which you’d likely not pay.

Committing two years of your life to AT&T, though, will drop the price by $100 to $149.99. Want more for less? A mail-in rebate via an AT&T promotion card scrapes off another 50 bucks.

With a two-year service contract and the mail-in rebate (after the weeks it’ll take to get to you), now you’re talking about snagging this premium fitness phone under the $100 barrier for $99.99. That’s generous, appropriate and a valuable opportunity for this feature-packed and innovative mobile phone.