I started with the same error and have got a little bit farther....!
Our ISA server was configured to use https for logon. The rule then converted from https traffic (incoming) to http traffic to the wss server.
I set the wss server to only allow SSL connections by modifying the default zone URL to https, then required SSL connections for this site in Internet Information Services manager.
I then deleted the publishing rule from ISA and re-created it using the Publish SharePoint site wizard.
On the Server Connection Security screen, ensure that the radio button "Use SSL to connect to the published web server or server farm" is selected.
Ensure you have set up Alternative Access Mappings correctly on the wss server, and when prompted by the ISA wizard, check the radio button for AAM is already configured.
This has fixed the problem for domain member devices accessing sharepoint via ISA (with Office 2007 installed) - everything works as expected.
An external check using a non domain member device with Office 2003 installed, the check in/check out buttons seem to do nothing at all!
HTH
Scott