just for the record, I solved this issue with help from the info posted on MS Knowledge Base
Basically, the problem lied on IIS, and it turns out that if you enable 'basic authentication' (i.e. clear text, SSL optional), you can login provided you have a valid Windows domain password (using DOMAIN\username).
I can now access MS SharePoint Services using either Opera, Firebird or Mozilla. IE-specific bells and whistles don't show up, but MS has implemented alternate code that will enable decent visualization on non-IE browsers.