We do not push major WordPress releases immediately. Major releases like 3.5 are typically feature enhancements whereas minor releases like 3.5.1 are security and bug related.
Minor Releases are pushed instantly.
Major releases are typically pushed within a 2-5 week window or at the first minor release the tag (3.5.1)
We delay for a couple reasons: Give plugin and theme authors more time to update their code to work with the new major releases, and it is the nature of software that a major release may have some bugs that are soon corrected in the next minor release.