Hmm, interesting. Android by default notifies apps that it's on a limited data connection, and they restrict background updates and such. For example, I can install apps from the Play Store on cellular, but they won't auto-update themselves. Same with apps like Pocket, Currents, Google Music (different bitrates for cellular and non-cellular backed wifi, pre-caching), etc. Gmail won't download images/attachments automatically on cellular, but will on wifi. For all of these though, you can usually override this in an individual app's settings, such as telling Google Music to always stream high quality or Gmail to always download attachments.

My tablet barely uses any data (with syncing enabled) in the background on cellular data. This despite having background syncing turned on and quite a few apps that use it. I take it with me most places and do casual browsing on the cellular connection (public transit and airports mostly), and the most I've used was 150MB/month. It just waits to do anything bandwidth intensive until it's back on wifi.

I'm really surprised Apple hasn't done this yet. I guess that's the reason why carriers complain iDevices use so much data.