You can only use 3G to access Amazon and Wikipedia. Wi-Fi is required to connect to other websites. I connected to 3G, and wasn't able to access
www.readerdailies.com, I turned Wi-Fi on and was able to.
Note: When using the experimental web browser, you'll only be able to access Amazon and Wikipedia using the 3G connection. Unrestricted web access is available by connecting your Kindle Paperwhite to a Wi-Fi connection.