Kindle with 3G + wifi already does exactly what you want automatically. When the Kindle connects to a wifi network the 3G turns off, ie. when you connect to your home wifi.
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For Kindle Touch models that include Free 3G, wireless connectivity is automatic (with no monthly fees or annual contracts) so if you see one of the 3G network indicators (3G, EDGE, or GPRS) in the upper right corner of your Kindle Touch screen, your Kindle Touch is already connected wirelessly using 3G. Your Kindle Touch automatically turns 3G coverage off when you connect using Wi-Fi. If you disconnect from a Wi-Fi network or if you move out of Wi-Fi range, Kindle Touch automatically switches back to 3G coverage. If you want to turn off 3G coverage, you can turn wireless off. Keep in mind that turning wireless off also disables Wi-Fi connections.
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