Ubuntu's need to catch a wave
Let me play devil's advocate here. Mark Shuttleworth's recent pledge to join a synchronised release plan for Enterprise Linux distributions is no more than a wish to benefit from a lot of work that Novell and Red Hat are already doing in the Enterprise space.
Let me explain.
Red Hat's Enterprise Linux offering is a very important proposition to businesses, not only because Red Hat guarantees (and has proven) to support each RHEL version for 7 years after general availability, but mostly because each version is updated with new hardware support every 6 months.
Why is that important ?
When a version comes out all components that comprise a distribution have been frozen half a year before and have been tested very hard. But hardware does not freeze and companies buy new hardware constantly. Red Hat needs to provide support for newer (selected) hardware from vendors to make its solution acceptable to customers. This work is not taken lightly. Red Hat has to backport fixes and backport kernel infrastructure, update drivers, perform regression tests and provide QA together with hardware vendors.
Every 6 months a new update release is being made to ensure that newer hardware is ready to be deployed with a recent RHEL version. Red Hat guarantees that new boot media is available with newer hardware support for another 3 years. That means 2 RHEL releases at every point in time are being maintained to support new hardware.
So what are you getting at ?
The sheer manpower to do this, together with new development and bugfixing for 4 different RHEL versions is something that Canonical/Ubuntu simply cannot take upon it. Even Novell does not support that many released versions in the way Red Hat does.
So Mark's article is wishful thinking and hoping to ride the wave that Red Hat (and Novell) are funding. If he can use that same kernel, with the same backports, fixes and regressions tests, Ubuntu LTS does not need to do anything to support the same vendor hardware. Easy, but at the expense of both Novell and Red Hat.
That explains why Mark wants 2 out of 3 Enterprise Linux distributions to enter his circle. I could imagine Novell and Mandrake joining forces to align the release cycle to try and take on Red Hat's 80% Enterprise Linux market share (sales, not necessarily install-base). There is hardly any benefit in Mark's proposal for Red Hat.
Also, Mark's Ubuntu timeline also resembles CentOS's timeline. And I guess he must have been studying Red Hat's release cycle very hard to come to his conclusion. (CentOS' timeline obviously comes from Red Hat's release dates)
Let me add that CentOS is not really involved in this discussion as CentOS does not directly compete with Red Hat (RHEL) or Canonical (Ubuntu LTS) in this market segment. Both offer paid-for services and support, while CentOS is provided as-is.
Update: I updated the CentOS timeline to include the upcoming RHEL6/CentOS-6 release.