Considering the way Safari caches pages, you're lucky if you're not forced to reload a page when switching between Safari windows, let alone apps. The Safari cache size on iOS devices is cripplingly nonexistent, and I wouldn't be surprised if it only used RAM and no disk caching.
The Gmail web app uses regular cache manifest offline functionality to cache the assets, but then uses Web SQL (of which the W3C has dropped the development of the spec) to store and serve up the messages.
