I have always thought, that accessing Sharepoint in Explorer View used WebDAV to do the work. But that is not always the case. WebDAV is actually quite restricted in how it can be used: It is limited to port 80 and consequently it can also not be used with SSL. So what is used instead? A protocol called FPRPC - FrontPage Server Extensions Remote Procedure Call. This protocol - designed before the standard WebDAV protocol was defined - is being used in all other situations. How can you tell the difference? Well WebDAV is a built-in network provider in Windows XP and you can map network drives to a WebDAV URL. Any Windows program can use WebDAV to access data. FPRPC on the other hand is a shared library only being used by programs taking advantage of it. This is typically the Office products. Windows Explorer is able to use both protocols, and in Explorer View you can tell the difference by looking at how Windowish it looks. 3D-icons and all the normal choices in the new menu signals WebDAV. A more flat view is FPRPC (this may be different in Vista).
Find you more in the whitepaper - Understanding and Troubleshooting the SharePoint Explorer View.