So does Robert Scoble's revelation, that Podtech is earning more than $400,000 a year from his show, prove the economic viability of podcasting? The former Microsoft evangelist, who now produces corporate videos for Podtech, told a conference that his online show received at least that much from sponsors. But it would be a big mistake to conclude, from Scoble's numbers, that the Bay Area podcasting company is sustainable. For a start, it looks as though a large part of the advertising is going straight to the talent; second, Scoble is the only star of Podtech's network; third, media properties like Podtech and Second Life are about as substantial as castles in a mirage. Even the most fad-following of marketers will eventually realize how tiny the audiences are for these corporate videos, and virtual showrooms. Robert Scoble should enjoy the bonanza while it lasts.