We use a two digit scheme, where every new release increases the number with 0.1. After 0.9 we go to 1.0. Test releases have a betaX suffix, where X is replaced by the beta release number. If the test releases are getting ready to become the next stable release, the rcX suffix is used.
Also see http://www.inliniac.net/blog/2008/09/22/new-vuurmuur-version-numbering-scheme.html about this change that was made after 0.5.74 alpha 6.