You know it's a bubble when... One of the most irritating features of the current web boom in Silicon Valley: the endless analysis of whether it's less a boom, and more a bubble. Witty observations, which all begin so: You know it's a bubble when... Fill in the gap. The exercise is repetitive. The discussion has all the fury, and significance, of a medieval debate about the number of angels who can dance on the end of a pin. And, worst of all, the discussion isn't really serious.Say one concludes that, yes, Lou Reed's performance at last week's Web 2.0 conference is incontrovertible evidence of bubbliness. Then what? Is it really as if any of the corridor cynics at internet conferences change their behavior? They're like alcoholics who go to meetings, but still drink. Or internet analysts, during the last cycle, aware that their buy recommendations were ludicrous, but unable to buck their system, or resist the bonuses that were in prospect.