“The real question is, What is a PC?”

That’s what Microsoft (MSFT) CEO Steve Ballmer said yesterday in response to a proclamation by the other Steve — Apple (AAPL) CEO Jobs — that the era of the PC is over.

But that was just one of lots of interesting questions lobbed at this week’s All Things DIgital confab in California…

What is a TV show in a multimedia marketplace?

What is journalism in an increasingly news-for-free environment?

And what, pray tell, is the media platform of the moment?

The last question has the clearest answer. Right after DreamWorks Animation (DWK) CEO Jeffrey Katzenberg declared his devotion to Apple’s iPad, FCC Commissioner Julius Genachowski said that on a recent trip to India, he toted a laptop, an iPad, and a Kindle. “I didn’t open my laptop once,” he admitted.

Half the D attendees were iPad-clad, it seemed. And as the masses have bought more than two million of Apple’s tablets already, content creators are finding they must get a new grip.

Like Lloyd Braun, who once co-headed ABC Entertainment for Disney (DIS) and then worked at Yahoo (YHOO). “Shows are not going to be just television shows,” he said at D.”They’re going to be brands.” Now partnered with former Fox TV boss Gail Berman in a firm they call  BermanBraun, he’s creating TV shows and web content with a much broader mindset than he’s ever had. “Television is just a marketing opportunity for the brand,” he believes.

With change comes angst, of course. Hollywood director James Cameron told the D crowd that his movie, Avatar, isn’t just the highest-grossing film in history; it’s also the most pirated film ever. The key culprits are young people, who happily view Avatar on an iPad or iPod or other tiny screen. “If it’s too small, you’re too old, I guess,” Cameron joked, regretfully, about his own dying generation of movie-goers devoted to the big screen.

Free news and information, meanwhile, threaten traditional journalism. Rich Rosenblatt, who scored big a few years ago selling MySpace to Rupert Murdoch’s News Corp. (NWS), now runs a company, Demand Media, that uses 10,000 freelancers to create 7,000 pieces of content–articles and videos, that is–on the cheap everyday. “We’re  part of the evolving ecosystem,” Rosenblatt said proudly.

As for Ballmer, the always boisterous Microsoft boss was obviously annoyed that Jobs and the iPad were the D darlings this year. “I think people are going to be using PCs for many years to come,” Ballmer insisted. He added that by docking an iPad onto a keyboard, you essentially transform the tablet into a desktop computer.

Fair enough, but that doesn’t gain Ballmer the edge he needs. His unpleasant reality is that Apple’s stock-market value now exceeds Microsoft’s. Of course he’s not relenting. “The race is on,” he boomed, rubbing his hands together.