Looks like we have a new bell (or is it a whistle) with J3.1b offering tags, not just any ole tags but nested tags with unlimited children! Yay!

community.joomla.org/blogs/community/1729-tag-you-are-it.html
"Tagging is integrated into all core components
Extensions developers can easily add tagging to their extensions
Tags are shared between extensions that implement tagging and across content types.
Tags are able to be nested. Each tag can have one parent tag and many children tags
There are currently a few layouts (with several options) and modules which you can use
Tags do not change the content that is tagged in regards to published state or ACL
"Multiple Categories" is #7 on the ideas.joomla.org site. This feature answers that request, albeit in a slightly different (and more flexible) way. For example, you can assign the same tag to different content types (which is not possible with categories) and then create blog views by tag."

Soo, when do I get to play with these delights in K2? :)

Hmm, no say?

Doubt you will see that any time soon since K2 comes with it's own tagging system.

I would love to see the K2 tag feature updated to what Joomla! 3.1 has. The 3.1 Tags still don't work correctly for me, but an advanced tagging (parents and children) would allow me to do exactly what I need to for one of the projects I am working on :)