For data, 10MB will be NOTHING.. you don't notice it on your Nokia because you're browsing WAP sites and they all keep data to a minimum. If you're using the iPhone, you'll blow past that in one day surfing the web because it uses a normal Safari web browser, not the hampred mobile ones loading tiny sites. Heck one page of CP can be 350KB. The page without images alone is 150KB. 66 refreshes, and you're done your 10MB
Of course there's the wifi which is good so if you are disciplined and stick to wifi only unless it's a dire need then that'd be one way to mitigate that.
Map function works by cell site sector so it's just a general locate not a GPS locate, not familiar with blackberry maps but I think the iPhone uses Google maps so it should be similar.
I've heard the latest jailbreaks are quite complete but there's always going to be the cat and mouse game of updates vs. cracks, as you say you can just not upgrade until you know it's safe.
The rumour is that new data plans are coming from Rogers in May (rumours on Howardforums) and 3G iPhones are coming in June so I'm waiting before I jump.. either for the iPhone to be officially launched in Canada, or at least until I can get a 3G version (which has much faster data transfer speeds) and a better data plan.