There’s a little-known stipulation in Amazon’s 3G browsing, available primarily on their e-ink devices. Ostensibly, downloading items over 3G is completely free but browsing the web using the device’s weird and slow experimental interface is capped at 50MB. Most users have never hit that cap and there haven’t been many reports of actual notifications.
That’s recently changed. One user of the Kindle Keyboard 3G noticed the message when he was browsing the web in Canada. He received a message that said he could only browse Amazon.com, Wikipedia, and the Kindle Store. Wi-Fi access was unaffected.