Heather posted a SharePoint questionnaire to determine what customers need, WSS, SPS or Both. Nice checklist when looking at SharePoint functionality.
"The following is a list of questions we ask prospective SharePoint site owners in order to start requirements gathering and determining which product implementation will best suit their needs. The questions help determine if the user needs a portal, a WSS site, or a combination of the two."
Take a look at the questionnaire.