I just Googled to find out more about this. Several sources report that as of July 1st Kindle Keyboard 3G web browsing is limited to 50 MB per month in some areas outside the United States. The only information I could find on Amazon was this, from Using Kindle Keyboard if You Live Outside the United States
Experimental Web Browser
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.