Outlook was playing with this in 1998 (via an ActiveX object in IE5). By 2000, Outlook was getting recognition for its use. In 2002, Oddpost was using it (later picked up by Yahoo). In 2004, Google was the one that deployed it using standards, and it was cross-browser on top of that. I think when developers realized it could be cross-browser and standards-compliant, they finally bought into it.