Amazon.com has quietly introduced a 50 megabyte usage cap on Kindle owners using 3G-equipped models to browse web pages over AT&T’s 3G wireless network. Customers exceeding the limit after July 1 reportedly began receiving this pop-up message on their device:

The Experimental Web Browser is currently only available for some customers outside of the United States and may be limited to 50MB of browsing over 3G per month. This limit does not apply when customers are browsing over Wi-Fi.

The new usage cap does not affect users browsing Amazon.com, Wikipedia, and the Kindle store.

Web browsing on an electronic ink display instinctively has a built-in cap: the limited patience of the user trying to browse websites that were never designed for the Kindle experience.

But some enterprising hackers managed to jailbreak the Kindle device and turn its free 3G connection into a wireless mobile hotspot.

That means Amazon was footing the bill for Kindle owners who have re-purposed the device to provide Internet connectivity to wireless phones, laptops, tablets, and other Wi-Fi enabled devices.

At AT&T’s prices, Amazon decided to pull the plug after 50MB, which is barely enough for a few dozen busy web pages accessed during the month.