Refining my earlier thought about tagging's secret sauce, tagging's popularity doesn't come from taxonomizing, it derives from signalling. By applying markers that can be monitored to media (URLs, photos) you can signal to other like minded folk the availability or applicability of that media, for a given community.
Technorati's tag search provides yet another means by which the signal is transmitted. Given Technorati's hig profile, tagging blog posts could become quite popular, but we'll need a better set of watch tools for exploiting the results. Here's hoping PubSub adds tag selection to their query language.