I'm moving from an iPhone 3GS to an Android device on AT&T. Been assured by two, different AT&T cust svc reps via email that I will be grandfathered into my existing unlimited data plan as long as I upgrade to any smartphone.
The issue is, I'd much rather purchase the upgrade from somewhere like Amazon Wireless vs. AT&T, since the price is quite a bit lower. But I've come across a few horror stories from people like me who were shoehorned into new data plans with the new device and spent a fair amount of time & frustration getting their old data plan back.
The problem with upgrading through a third party is that they may not be able to specify the unlimited data plan since it is no longer offered by AT&T. If you contact AT&T customer service immediately after upgrading and have them verify that you're keeping your unlimited data, you shouldn't have a problem. Other than having to deal with AT&T customer service, of course.
i have also herd those horror stories about amazon some of them have been able to get there unlimited back by calling customer service with att. So if u decide to go amazon's route make sure to call. i personally went with best buy because they price match any local in-store price around there (meaning if att, walmart, target, or any other retailer have a cheaper price around you they will match it).