More than 20 years ago, Apple envisioned the Knowledge Navigator in the form of a tablet computer. Using hypertext, video conferencing, and artificial intelligence (AI) made the Knowledge Navigator a perfect personal assistant. The tablet form factor of the iPad (and maybe even the iPhone) would be ideal, but the technology just isn't possible, yet.
The key to the Knowledge Navigator is its AI. Advances in this area are slow due to the complexity of mimicking a human and getting it to pass the Turing Test. Keep in mind that it took mother-nature more than four billion years to create human intelligence.