What you are asking for is arguably a very specific feature and something that is probably too complicated for typical end-users. But it is nonetheless quite useful and something that should in fact be available for power users who don't get confused by the fact that they might accidentally disable all data-communications on their device.

On GSM/UMTS devices, you can probably do most of what you want to do by installing APNDroid from the Android market. It allows you to selectively disable non-Wifi data connections. There are a few other applications (e.g. Timeriffic) that are aware of APNDroid and that can be configured to en-/disable data based on other factors (e.g. time of day). I believe there are similar applications that can trigger based on location, but I haven't tried that myself.

Unfortunately, I believe that this option is not really available for CDMA devices such as the Droid. Last I checked, APNDroid only worked on devices that actually use APNs -- and that's a GSM/UMTS technology.

Normally, in a situation like this, I would suggest you look at community-firmware such as the Cyanogen project, as they tend to be quite good about adding a lot of features for power users. But again, I think that won't help you. Last I checked, the Droid was difficult to work with and didn't have much in the way of community-firmware.

Sorry that I can't be of more help. But maybe some of these suggestions will give you an idea on where to look for a solution.

Android phones will use Wifi if available and only use 3G for existing TCP connections. The latter should be far and few between, and they are unlikely to use much data. You'd only have those if you started a connection on 3G and then walked into an area that has Wifi.

By default, Android phones disable Wifi when the phone is not in active use. So, background applications can still incur some 3G usage. This setting made sense on older hardware (e.g. G1, ADP) which needed a lot of power to keep Wifi connections active. Having Wifi permanently turned on would result in sharply reduced battery life.

For more recent Android hardware (e.g. Nexus One), it is generally a good idea to change the Wifi "sleep policy" to never. This will keep Wifi active even when the screen is turned off. And in fact, it often results in better battery life. It turns out, on these devices Wifi is a lot more efficient than 3G. So, not only are you conserving your data quota, but also your battery power.