It has been a while since I was last in this thread so I thought I'd update it.

SharePoint conforms to WebDAV so there is a special version of a UNC to be used to map a drive letter.

The following example uses SSL

Use u: "\\spad.mysite.com@@SSL\DavWWWRoot\share"

The following is for any WebDAV enabled site. I'll use Sysinternals as an example since it can easily be demonstrated.

use u: "\\live.sysinternals.com\DavWWWRoot"

Tested over the years and its a great way to deal with SharePoint or any WebDAV compliant server.