Please don't do that! That would be so annyoing. I'm fine with the current method, where you just enable/disable 3G/Wifi connection via the quickmenu on top of the homescreens. I see no problem with that, being connected to the internet 24/7 is just useful for messenger and skype anyway. Messenger could run with 2G. So your friends telling you in skype chat, that he wants to call you, you enable 3G in quick menu, or maybe with a keyboard shortcut or something similiar and then you call him. I think we need no automatic switching, since manual switching is that easy, but as an optional battery life saving method, it's ofc very welcomed.