A &34'bridge&34' app allows the new BlackBerry PlayBook tablet to connect to a BlackBerry smartphone to access e-mail and a 3G data connection.

AT&T may be putting the kibosh on an application for BlackBerry smartphones that allows users to connect their new BlackBerry PlayBook tablets to AT&T's 3G wireless network for free.

AT&T has confirmed that the app that allows BlackBerry users to pair their RIM smartphones with the new PlayBook tablet is not currently available on AT&T BlackBerry phones, a fact that limits the functionality of the device for AT&T BlackBerry customers.

AT&T said the app isn't supported yet on its network because it hasn't fully tested the app. But some people suspect that AT&T may be unhappy with how the app allows BlackBerry users to bypass its $20 tethering fee.

"We are working with RIM to offer the 'bridge' app to AT&T customers," AT&T spokesman Mark Siegel said. "We've only just received the app and we're testing it now to make sure it will deliver a great experience to our customers."

The "bridge" app, as RIM calls it, allows PlayBook users to pair their tablets with their BlackBerry smartphones. In essence it lets them get their BlackBerry e-mails, calendars, tasks, and contacts. It also offers access to the BlackBerry Messenger service. When the devices are separated, all of that potentially sensitive content stays on the phone and isn't left on the tablet. RIM says this is an important security feature for corporate IT managers.

One of the perks of the bridge app is that it allows users to tether their BlackBerry smartphones to the PlayBook to get 3G wireless data access. The initial version of the PlayBook, which went on sale today, only offers Wi-Fi access.

Last week at the launch party of the new PlayBook in New York City, RIM co-CEO Jim Balsillie said that the "bridging" app for the PlayBook was a big differentiator for the Playbook over either the iPad or other Android tablets that are sold through carriers, because the "bridge" app allows BlackBerry users to get 3G data access on their tablets without paying an additional fee. AT&T charges $20 for tethering, which is in addition to the $25 2GB data plan for the smartphone data plan. Verizon also charges an additional $20 a month for 2GB of data to tether a tablet to its smartphones. This is in addition to the $30 a month data plan that's required for the smartphone.

"Why would I pay an additional $30 a month to tether when I can use my BlackBerry with the PlayBook" he said.

AT&T recently began cracking down on customers who were tethering their smartphones to laptops and tablets without paying the extra fee. The company has been sending e-mails to customers who they suspect of tethering without paying the extra fee, warning them that they will be charged if they don't sign up for the service themselves.

Siegel said last month that AT&T is able to detect how devices are being used. But he wouldn't explain how the company determines whether a smartphone is being used to connect to the Internet or whether it's used to provide Internet connectivity to other devices.

But as Balsillie noted last week, one of the good things about the PlayBook from a consumer standpoint is that when the "bridge" app is used it essentially merges the two devices so that they're theoretically seen as one on the network. This means that AT&T or any other carrier wouldn't be able to detect that a BlackBerry is being used to tether to the PlayBook.

AT&T's Siegel would not comment on whether AT&T was preventing the app from being downloaded on their BlackBerry phones as a way to prevent users from tethering their devices.

"We do offer tethering plans for smartphone customers," he said. "But we can't speculate on how that applies or doesn't apply here until we have tested the software."

News that AT&T BlackBerry phones could not use the "BlackBerry Bridge" was first reported on the Web site Crackberry. My colleague Nicole Lee from CNET Reviews has posted a work-around for the issue to allow AT&T customers to still download the app. RIM did not return requests for comment.

RIM indicated on the Web page where the "bridge" app can be downloaded from the BlackBerry App World market, that the app is supported on a slew of BlackBerry phones on all U.S. carriers except AT&T. Still other wireless operators were a bit cagey when asked about the free tethering feature. Verizon Wireless and Sprint Nextel, which plans to offer a 4G version of the PlayBook this summer, were unwilling to comment further on whether they approve of BlackBerry users being able to bypass tethering charges.

"We have announced one PlayBook--BlackBerry 4G Playbook--but not gone into specifics on the bridging app or any other features," Michelle Leff Mermelstein, a Sprint spokeswoman said via e-mail.

Verizon Wireless also wouldn't directly answer questions about how BlackBerry tethering for the PlayBook may be treated differently than tethering for other smartphones and tablets on Verizon's network. Verizon has not announced plans to offer the PlayBook.

In the past, Sprint Nextel and Verizon Wireless have blocked tethering on Android smartphones. For example, my colleague Kent German of CNET Reviews points out that when the Froyo version of Android came out for the Motorola Droid and HTC Evo, Sprint and Verizon deleted the feature for those devices. And AT&T has also historically not been a fan of tethering. When tethering became available for the Apple iPhone in 2009, AT&T waited more than a year to offer the capability for iPhones on its network. AT&T didn't allow tethering for the iPhone until the iPhone 4 was launched in June.

For now, at least, it looks like BlackBerry users on Sprint, T-Mobile USA, and Verizon Wireless will be able to benefit from the free 3G wireless access. And AT&T BlackBerry customers will have to wait.