Aaron Elder of invokesystems has just posted a fine article breaking down the version numbers that Microsoft uses for their products and in particular the CRM product line.
"The number is broken down into parts, which are separated by a period. The format is as follows:
[MAJOR VERSION].[MINOR VERSION].[BUILD NUMBER].[REVISION NUMBER]
In the case of Microsoft CRM, the current version is 3; there is no minor version and the build number that was finally shipped ended up being 5300. The “revision number” is generally used for hot fixes and the patches. Once a product like CRM ships, the code tree is “forked” (copied) and all hot fixes are applied to this branch of the code tree. The “main branch” continues on and becomes CRM 4.0… or whatever."