You will be able to convert a WiFi-only iPad tablet into a pseudo 3G model thanks to a $10 App that converts a jail-broken iPhone smartphone into a WiFi hotspot to which an iPad (and in theory other devices) can connect seamlessly.
Appadvice (opens in new tab), which originally reported on the application, says that MyWi is available with a 10 day trial when downloaded through Rock rather than Cydia.
They also conducted a test and found the overall speed to be "excellent" although using the iPhone's 3G capacity may quickly drain its battery and make it rather warm.
The upside though is that you'll end up paying less for the WiFi in the first place and will end up using your iPhone unlimited data contract as it is the case on O2 in the UK and AT&T in the US.
This obviously violates the terms and conditions of both networks that specifically prohibit all kinds of tethering but then Apple forbids jailbreaking its iPhone or face the risk of eternal damnation.
MyWi uses 40 bit and 104 bit WEP Security to protect access to the network and allows USB and Bluetooth Tethering on the target iPhone as well.
It reportedly uses less battery and is much faster than competing solutions due to the fact that it implements native routing techniques.