At OpenAI's DevDay this week, OpenAI boss Sam Altman did what American tech bosses rarely do these days: he actually answered questions from reporters. "I know it's tempting to write the bubble story," Mr Altman told me as he sat flanked by his top lieutenants. "In fact, there are many parts of AI that I think are kind of bubbly right now." In Silicon Valley, the debate over whether AI companies are overvalued has taken on a new urgency. Sceptics are privately - and some now publicly - asking