Please post IIS6 questions in the IIS6 forums in the future.
On IIS6 also, .htm/.html go to the static-file handler - if you want server-side include functionality for it, you need to map it ssinc.dll (the same that .shtml is mapped to) - I do not have a IIS6 install handy, but there is a UI to configure scriptmappings somewhere
On IIS7, just use the following command
%windir%\system32\inetsrv\appcmd.exe set config -section:handlers -+[name='SSINC-htm',path='*.htm',verb='GET,POST',modules='ServerSideIncludeModule',resourceType='File']
%windir%\system32\inetsrv\appcmd.exe set config -section:handlers -+[name='SSINC-html',path='*.html',verb='GET,POST',modules='ServerSideIncludeModule',resourceType='File']
the same can be accomplished from the handlers UI