You can still access Amazon content on WiFi, and they could play some games with caching. Flash is sold at a premium (wasn't the iPhone like, $100 to go from 8GB to 16GB?) but not actually all that expensive.

Google Music is a perfect example, actually. I allow it to stream music over 3G, but it still aggressively manages a local cache of music anywhere from 1-2GB, leveraging WiFi to save data. (It would use more if I didn't have a dinky 8GB device)