Jailbreaking the ipad may allow you to run unsigned code and do all the funky mods but it also removes all security mechanisms and code checks by the vendor. (not that it needs jailbreaking to run unsigned code as we all know.. but that's for a different thread).
Maybe research the market for a 3g to wifi little bridge.
Extending and Protecting 3G/4G Networks Wirelessly | Cradlepoint Technology have portable solutions that work with blackberry. That means sadly another thing to carry and charge but they're pretty small and can fit in the smallest of laptop bags. Then you just hook it on the BB and connect your devices (ipad, etc) on it over wifi.
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