Nick - the cache lasts until the new map is released. I wouldn't call it "cache" but rather a "download."
You don't have to browse the map beforehand (like Google earth), you simply go into the download section of the app and choose the relevant VTC / VNC etc. Those are roughly 12-13 MB each. The WAC is a one-piece download somewhere in the mid 200 mb's. You can download all VNC's etc or none, your choice. If you're on a good connection it's pretty quick, or drop by McDonalds on the way home from work.
Yes, indeed you can zoom all of the maps offline, assuming they have been pre-downloaded.
As someone that knows a little about IT, the only risk I see with it is when there are thousands of subscribers and it ticks over the new map period, everyone will hit the downloads page and bring the OzRunways ISP down, but I'm assured that the boys at OzRunways have taken this into consideration and increased their download limits
