You are able to upload Visio files to a document library in SharePoint and view these files directly from SharePoint using the Visio Viewer. WSS has a demo of the Visio Vewer in a Content Editor web part here:
http://www.wssdemo.com/Pages/visio.aspx
The Viseo Viewer can be downloaded directly from Microsoft:
http://www.microsoft.com/downloads/details.aspx?familyid=d88e4542-b174-4198-ae31-6884e9edd524&displaylang=en
Integration with SharePoint includes opening Visio documents from SharePoint directly in Visio (if the user has the full Visio client or in the Visio Viewer if the user does not). It is also possible to access documents on a SharePoint site directly from Visio and view SharePoint task lists and calendar lists as Visio diagrams.
MSDN has a very thorough article on Integrating Visio 2007 and SharePoint Products and Technologies detailing these features.
http://msdn2.microsoft.com/en-gb/library/bb229690.aspx
