Locale powersave plugin & Power Control widget
I am the developer of Locale.

To get started, please read this article on our website about toggling 3G which should answer your questions: http://feedback.twofortyfouram.com/forums/5655-locale-feedback/suggestions/67867-3g-support-for-network-setting?ref=title

From that article, I think you'll find that using APNDroid will be the best solution: it is free, has a Locale plug-in, and it contains a widget that can be used to manually toggle Internet access on the phone.