From the "whatever happened to the dewey decimal system" files:

Linux kernel developer Greg Kroah-Hartman is leading an effort now that could possibly end up changing the way the Linux kernel is numbered.

Currently kernels are on an x,y,z scheme where the latest point release is 2.6.27 while the bug fixes and security updates are 2.6.27.1. Kroah-Hartman is proposing a calendar based system that will help identify the aging of the kernel.

In a posting to the Linux Kernel Mailing List, Kroah-Hartman wrote:

So, as someone who constantly is dealing with kernel version numbers all the time with the -stable trees, our current numbering scheme is a pain a times. How about this proposal instead?
We number the kernel based on the year, and the numbers of releases we have done this year: YEAR.NUMBER.MINOR_RELEASE

Under Kroah-Hartman's proposal the first release of 2009 would be 2009.0.0.

I haven't yet seen Andrew Morton or Linus Torvalds weigh in on an opinion on this either way. In my own personal opinion, I can certainly see benefits in a calendar approach and as a journalist (rather than as a user) it would save me the grief of looking up when a kernel came out.  As a user, frankly, I don't care. The current numbering system is logical and systematic so I have no problems with it.

That said, every Linux vendor that doesn't use kernel.org kernels always appends the suffix of a kernel number a bit anyways and that's where I tend to get confused.