I was recently working on a custom Drupal 6.x module that needs to create a custom content type with fields upon installation. I did quite a bit of hunting for how best to do this. There are a few different approaches, including using the Features module, or having my module do all the node and db handling, etc. I decided to use the export and import features of CCK, and it's very easy to do. Here is a link to the handbook page at DO that explains how to do it.