Mike Fitzmaurice is talking about the SharePad project which can be found on GotDotNet. If you want to add document library support for your own editor you will have to use the FrontPage RPCs according to Mike. As stated on the website of GotDotNet:

"SharePad is a reference application demonstrating Microsoft Office-style SharePoint integration techniques. This application illustrates the use of the Front Page RPC calls exposed by SharePoint among other SharePoint integration APIs."

The following functionality is available: