1TB is not impossible to achieve, but really almost impossible. At least within 5-7 years.
Why? Currently in the market today, even a 246GB SSD for desktop and laptops are expensive configurations for now. Most would simply just either opt for a hybrid or a normal hard drive. Of course, cost will go down as years go by. But as for now in 2012, a 256GB SSD will cost u maybe nearly 1/3 of the price of a decent 1k computer for a small component of your computer bundle.
The iPhone will not have a 1TB SSD. Why?
1. If the iPhone needs an 1TB SSD, then I suppose the desktop with much more functionalities and demand from its users would demand maybe a whopping 1000TB. Just stating a comparison. A 1TB SSD has no absolute functionality over the fact that is has space and a big one. An iPhone is meant for listening to music, playing angry birds, taking a 1080p video (Which is by the way, can't replace your real camera yet,face the fact!) The iPhone does not serve as a external hard drive and storage, but is mean for media consumption on the go. A phone will never replace a computer, vice versa. Thats what a tablet is supposed to aim to achieve, not the phone. I would not strike out the possibility of a 1TB storage on the iPad though.
2. Unless of technological advancements, a 256GB SSD is still bigger than the iPhone. Technological advancements that is supposed to make this 256GB SSD shrink and fit into the size of the iPhone is not possible. Unless you can find alternatives to the silicon material which all electronics are using which is better of course.
2. A too large SSD in an iPhone may be detrimental instead of beneficial. 1TB SSD may cause its users to dump everything on the phone with no absolute control (Just like how some users treat their computers). Booting up the 1TB filled iPhone will be an issue as it will be a long wait. Also, a small device does not have comparable performance compared to a desktop class or real computers in general/