Primary support involves being able to pick up the phone for basic help, primarily, and patches of both security and a bug fix nature.
Extended support involves the things of primary support but are costly. Security patches are released, generally, but any other problem involves licensure and payment for the fix (Several SP1 and Windows 2000 boxes got hit for about $2000 a pop for the time zone fix last year).
Service Pack 3 RTM will be released on April 29, and will likely be the last one. So the issue of service packs will be irrelevant.