The theme of eternal consciousness through a computer or artificial life form is not limited to Star Trek. The Dr. Who episode Silence in the Library is about a girl trapped in a computer; it is a simulation created by her family to store her consciousness alongside all information in the universe. Literature, music, and art are to be her companions.
Steve Jobs knew he was dying but did not seek appropriate treatment; he distrusted modern medicine. But he had a tremendous amount of trust in himself. He worked on revolutionary new technology towards the end of his life. Jobs was also very cognizant of his impending death:
Remembering that I'll be dead soon is the most important tool I've ever encountered to help me make the big choices in life. Because almost everything — all external expectations, all pride, all fear of embarrassment or failure - these things just fall away in the face of death, leaving only what is truly important.
What is more important to a man than his own essence? Very few things. Steve had ample time to create a system that would capture what was truly important to him.
Is Steve Jobs in the iCloud?
The storage technology does perhaps exist for such a system.
The ability to read and transfer the mind's information almost certainly does not. This obstacle would be enough to prevent even the most technologically genius billionaire from uploading themselves to an artificial device. Steve was not a doctor, an his statement regarding his pancreatic cancer, "I didn't even know what a pancreas was" reveals that he most certainly did not have the medical background to create a secret piece of technology. If he hired someone to attempt such a feat, the news would have almost certainly leaked out.
More difficult than that would be the provisions for processing power. The mind functions differently than traditional computers, with many more parallel operations. The mind isn't necessarily faster than a computer, but it has more hauling power, the way a dump truck would compared to a NASCAR vehicle. It is difficult to believe that anyone would have the control technology to parallel enough processors to simulate a full human mind, even if we did understand the mind enough to design such a system (which we do not).
Steve Jobs most certainly does not live in the iCloud, but it is a testament to his visionary genius that such a theory is even being circulated.
Addendum: Since this was originally published, some have begun to claim that Siri stands for "Steve is really inside."