The current iPhone 4 has 16GB and 32GB capacities available. Apple's estimates, which are of course grossly exaggerated like any OEM's estimates, say that 32GB of storage gets you about 7,000 songs or 40 hours of continuous video. So just think of approximately double that for the 64GB version, with a higher price tag of course.
The highest rated comment on this video caught my eye: "The serial number indicates that this particular prototype was produced in early 2010. So, while Apple may be working on a 64GB iPhone in the future, this particular one was an old prototype that pre-dated the launch of the iPhone 4." The device is running iOS 4.1, which is not the latest version of Apple's mobile platform, so this further supports that this is an earlier prototype.
We don't think that Apple is preparing to sell a 64GB version of the iPhone 4. It may be revisiting the idea with the iPhone 5, however, which is set to debut this summer. Specifically, we're expecting Apple to release its latest and greatest, with a rumored larger screen, in June 2011.