While there’s no real master page concept in Windows SharePoint Services 2003, I’ve found that editing "C:Program FilesCommon FilesMicrosoft Sharedweb server extensions60TEMPLATE1033MPSdefault.aspx" to replace images, resize objects, etc. works much like a master page in SharePoint 2007 would.
Between the default.aspx file and the OWS.CSS file (C:Program FilesCommon FilesMicrosoft Sharedweb server extensions60TEMPLATELAYOUTS1033STYLES), I’m able to manipulate the global look and feel of my WSS site fairly easily. (But not without the awesome Internet Explorer Developer’s Toolbar!)
UPDATE: My colleague Mauro Cardarelli pointed out, quite correctly, that if you edit files in the file system, then you *MUST* document your changes for disaster recovery. It’s easy to make it all look nice, but if you don’t remember how to fix it or redo it, whoops. Bad stuff.