GMail's support for IMAP (the mail protocol that supports synchronization of folder-structure between mail clients and servers) Ultimately may push other email clients towards supporting GMail's "tagging" rather thant "foldering" model of email management.
And, once you have a generic "tagged" message synchronizing and managing architecture, why stop at email? What about other docs? The proliferation of other messaging protocols and streams (eg. Twitter, RSS, Jaiku, River of News etc.)?
Because of the way IMAP supports the nesting of folders, supporting IMAP meant that Google had to figure out a way to hierarchically nest labels. So, along with the introduction of IMAP support, Google has introduced the slash (”/”) as a means of telling GMail that one label is a parent to another. For example, in GMail, the parent-label “Northeast” could have child-labels as in “Northeast/Massachusetts” and “Northeast/Connecticut”. On the client side, such a hierarchical taxonomy would be reflected in parent and child folders with no limit to nesting depth.
The introduction of hierarchical tagging begged two more globally directed questions for Google; First, will the hierarchical tagging be made available in other Google services (ie: Google Reader, Google Docs, etc.). Second, when will (note, not “if”, but “when”) Google’s customers be able to unify their taxonomy across those other applications so that users can see everything that’s tagged with the same label (eg: “Northeast/Massachusetts”) regardless of what service it comes from.
Hierarchical tags ... that's like square circles. Except in this case it might be a felicitacious synthesis.