IMHO this is the best solution. Having the network connection pause when you first open a website would be damned annoying, and this should realize 99% of the power saving. One addition, though: forcibly keep the indicator on 3G so that the user doesn't always get distracted by the icon changing a few seconds after the screen is activated. The only downside I see is that users who constantly check their calendar would be draining their battery with reconnections. This could be solved with a minimum interval between successive switches to 2G.