Extensions will work if they don't use some API thingee (that's the technical term) that Google doesn't find secure enough. Don't have my notes, but NP/API? Anyway, most of the stuff in the market they tell me should be fine -- but even there, not all of it will be, guaranteed, I was told. In short, Chrome in Chrome OS is not exactly the same as the Chrome browser.
On iTunes, we'll all be using music from the cloud. Seriously -- my second day interview with them will get into this. Maybe so. But if you're into Apple devices, you're still going to need iTunes.
On 3G, I have a 5GB plan that I've used for years. Never gone over my limit. But, I only use it for when I'm traveling, about 5 days or so per month. If this was your main plan, expensive. But Google positions it as something you can use if you're out and away from WiFi.