The HTC Inspire, one of five AT&T Android phones that will now be able to sideload apps like Amazon's Appstore. Jim Merithew/Wired.com

After years of restrictions, AT&T will now allow Android smartphone customers to install applications downloaded outside the official Android Market.

The recently launched Samsung Infuse is the first of AT&T’s phones capable of installing apps from outside sources, including unofficial app stores or web links, a process called sideloading.

Eventually after some software updates, all AT&T Android smartphones will be capable of sideloading, according to AT&T.

“Over the next few weeks, we will also roll out this capability to existing devices in our base for which an upgrade is possible,” an AT&T spokesman said.

The HTC Inspire 4G, Samsung Captivate, HTC Aria and LG Thunder will receive the over-the-air upgrade. AT&T is also working on bringing this upgrade to the Motorola Atrix, although AT&T is waiting on a “future maintenance release” for the Atrix in order to upgrade the phone.

What AT&T isn’t saying, however, is that you most likely have Amazon to thank for this.

On March 22, Amazon released its Appstore for Android phones in the form of a standalone app. Those who wanted to access the 3,800 Android applications — including a new version of Angry Birds — from Amazon’s market were required to download the Amazon Appstore app from Amazon.com.

The problem was, AT&T’s phones wouldn’t let you do that. Users could only download apps available on the official Android Market, and because of Google’s policy of not allowing competing app stores to exist within the Android Market, Amazon’s Appstore wasn’t allowed in.

AT&T customers unable to access Amazon’s Appstore weren’t happy, and took to the forums to express frustration.

“Every day, Amazon releases a new app for free,” one upset customer wrote on AT&T’s own hosted message boards. “We continue to be left in the dark … Why can’t you just allow us to install 3rd party apps on our supposedly ‘open’ Android devices?”

On the same day as the Amazon Appstore announcement, however, AT&T changed its position. On March 22, AT&T told Wired.com, “We’re working to give our Android customers access to third-party application stores.” With the release of the Samsung Infuse this week, AT&T customers will finally have access to sideloaded apps.

Amazon confirmed to Wired.com that it had been in discussion with AT&T prior to the Amazon Appstore launch in March, and that Amazon is currently “working closely with AT&T to help make it as easy as possible for AT&T customers to have access to the Amazon Appstore for Android.”

On other non-AT&T Android phones where sideloading is allowed, users must check a box in the application settings menu for the phone “to allow installation of non-Market sources.” After checking the box, a small disclaimer pops up, letting you know it’s your fault if you download malware and screw up your phone. AT&T confirmed that a similar process will be available to the five listed AT&T phones in the coming weeks, as well as future AT&T Android phone releases.

According to an AT&T, the company wasn’t allowing sideloading because of security issues. In an interview with AllThingsDigital, AT&T CTO John Donovan said that “although there was a vocal minority clamoring for such access,” the vast majority of AT&T users prefer a secure phone more than one that can access any and every app out there.

“I’m a gigantic new services risk-taker,” Donovan told AllThingsDigital. “I’m not at all a risk-taker as it relates to security and privacy.”

A security researcher previously told Wired.com that allowing your phone to download apps from unofficial sources required some security compromises.

“As soon as you flip that switch and go away from the Android Market, which is the one place where most people go, then you are putting yourself at some risk,” security analyst Charlie Miller told Wired.com in a previous interview.

But with the release of Amazon’s Appstore, AT&T seems to be more of a “risk taker” than CTO Donovan let on. Maybe the clout of a multinational online retailer can make a company loosen up a bit, even if it supposedly means being less secure.