Am now doing research on how to make Site Definitions for MOSS 2007/WSS 3.0. The main reason to do this, is as far as I can now determine (this may change) still the same reason you made one for SPS 2003.

You can use Site Templates (which are entirely different animals) within any particular top level site/site collection by installing them after the top level site is running, and then base new subsites on those within the gallery of the site collection.

In contrast, if you want a thing that acts like a Site Template but applies to the root, top level site in a site collection while you are creating the site collection for the first time, you need to have already created and installed a site definition that looks/feels/behaves the way you want it to.

Anyhow, here are the how-to links:

• Serge van den Oever [Macaw]’s blog
  SharePoint Solution Generator – part 1: create site definition from an existing site
  Talks about using Visual Studio’s Beta WSS extensions to create a WSP file.
• SharePoint Solution Generator – part 2: the internals of the created site definition project
  Goes deep into what’s going on in ONET.XML and the WSP file.
• Blog: Tod Baginski’s SharePoint 2003 and MOSS 2007 Blog
  HOW TO: Creating a custom Site Definition in MOSS 2007
  Treats the barest essentials. Copy the site definition, write a new entry for it in ONET.XML, reset iis. Nothing about the actual work of customizing the site definition.
• MSDN – related topic – master pages
  Customizing Master Pages in Windows SharePoint Services
  Master Pages come to us via ASP.NET 2.0. I’m not yet clear about Best Practices in mixing/matching master pages and site definitions.
• Madhur Ahuja’s MOSS 2007 and Office 2007 Blog
  Site Definitions Demystified – Creating a custom site definition having Custom webparts
  Title is self-explanatory, looks very similar to what I’ve done in SPS 2003/WSS 2.0
• Robert Bogue [MVP]’s blog with no name?
  Custom Master Pages and Site Definitions in WSSv3 (and MOSS2007)
  Talks more specifically about some misconceptions about ONET.XML entries.
• Joel Oleson’s SharePoint Land
  Resources and Recommendations for Upgrading Site Definitions and Site Templates
  Talks specifically about updating site definitions from SharePoint 2003 to 2007. Many helpful links.
• devx.com’s article:
  SharePoint Development Just Got Simpler Thanks to Visual Studio Extensions
  Talks about Visual Studio extensions that make List Definitions and Site Definitions easier to write/convert.
• Shane Perran’s SharePoint Customization Blog
  Custom Site Definitions in MOSS 2007 (Tip!)
  Talks about creating a new tab in the site collection creation page for your site definition.
• WSS Demo (wssdemo.com)
  SharePoint Resources: Tips for creating Site Definitions (webtemp file)
  Some small but helpful tips.