NEW YORK ( TheStreet) -- Amazingly, nobody has yet pointed out a severe usage limitation on Apple's ( AAPL) iPad that surely will cause grief with the very first reviewers. I'll get right to the bottom line: Just like the iPod Touch, theiPhone and most or even all other smartphones, the iPad lacks multipleuser profile logins, including any "Guest" login.

Think of the iPad as one big iPod Touch or iPhone. Once you'veentered the password, you're in. And I mean in! You have completeaccess to all emails, instant messages, the address book and calendar.Contrast this with a laptop: On a PC, you may have, say, fourdifferent user logins (father, mother, son and daughter) and onegeneric "Guest" login. This means you can't see others' emails,instant messages, address books, calendars and any other documentscreated. Privacy is protected.

So why is the lack of multiple user login or a guest account such acritical flaw on the iPad, when the world of smartphones doesn't seemto have crumbled in the wake of a similar deficiency? The answershould be obvious, but I will spell it out anyway: Unlike an in-pocketsmartphone, the iPad is almost naturally a somewhat communal device.

Where will you find the iPad most times? On the table, not in thepocket. What is its main purpose? To surf the Web, among many otherthings, of course. There is almost an expectation that anyone shouldbe able to pick it up and use it. Just imagine the regular family offour: Residing on the dining table, kitchen counter or coffee table,the kids in the family will be jumping for the iPad at every moment.Will you tell them, "No?" Can you imagine their loud screams?

For the iPad to have meaningful utility to a productive adult, itneeds to be synchronized with your personalized data using iTunes andApple's MobileMe. But wait. By containing all this personalizeddata, including your emails, address book and instant messages, justfor starters, the kids will be only seconds away from destroying yourmost valuable information -- often work-related -- on the iPad. You cansee the headlines right now: "Kid Gets Password to iPad ...." Fill in the humiliating blank.

The fact of the matter is that the iPad is a lot more like a laptopthan a smartphone in terms of how you need to protect yourinformation. You wouldn't let your kids use your laptop under yourpersonal login, with access to your emails, address book, documents,and instant messages. This will force parents and others to not synctheir personal information -- through iTunes and MobileMe -- with theiPad, at which point the iPad has immediately lost a material portionof its intended utility.

It should be very easy for Apple to fix this critical flaw. In fact, Apple needs to do the same for the iPhone and the iPod Touch as well.You want to be able to lend your iPhone and iPod Touch to someone -- nomatter how temporarily -- knowing that all this person can do is tosurf the Web, depending on how limited you have set the permissions. Thesame goes for all the other smartphone systems - Blackberry from Research in Motion ( RIMM), Google's ( GOOG) Android, Palm ( PALM), Nokia ( NOK) and so forth.

Until such time that Apple fixes this critical flaw, however, thisgives rise to what I predict will be the biggest complaint about thedevice, with the possible exception of the much-feared lack ofubiquitous multitasking. As it stands, no user in his or her rightmind will dare to synch the iPad with iTunes and MobileMe. And theregoes much of the utility of the device. And for those who dare, theyhave just signed their own personal information death warrant courtesy of their kids.

Someone told me that the solution will be for that family of four tosimply buy four iPads. That may even be Apple's secret plan to beatall of its revenue estimates! Please. If there was a more obviousway to upset your new iPad owners, I can't think of one. You havejust spent $500 to $830 on a device that's fundamentally less capablethan a $300 laptop and immediately you're told that you need to buyfour of them in order to protect your personal information. Please!

In the movie "Armageddon, one astronomer declares "We have 18days!" until an asteroid hits Earth. Indeed, Apple, you have a April 3 deadline to fix this. You can do it. I hope. Or else the reviewswon't read so well.

At the time of publication, Wahlman was long Apple, Palm, Research In Motion and Google.

At the time of publication, Wahlman was long Apple, Palm, Research in Motion and Google.

Anton Wahlman was a sell-side equity research analyst covering the communications technology industries from 1996 to 2008: UBS 1996-2002, Needham & Company 2002-2006, and ThinkEquity 2006-2008.

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