Currently K2 does not allow punctuation marks to be used in the tags...Now, is that important? I would say that depending of the type of website that one uses then it can be a real problem...
let me describe it with a couple of examples..

Punctuation marks could be needed for example
- If the words/names require their usage, e.g. for a news site, one would need to use "Jean-Claude Trichet"
- In order to differentiate between terms that refer to different things but spell the same, e.g. initials that correspond to different institutions, groups etc, so that the use of punctuation marks allow their differentiation...e.g. in Greek the initials ELAS (ΕΛΑΣ) may refer to a) the greek police force (ΕΛ.ΑΣ) or the greek popular liberation army (Ε.Λ.Α.Σ), from the mid-20th century...

Lack of differentiation here, therefore unlikely groupings of articles in tag listing hinder the reading experience of website visitors who might be looking for specific things to read.

I have asked this before and the technical answer that I got was

Joomla! JRouter class replaces the first hyphen with a colon character assuming that the first part is the id variable. Because K2 is built in such a way that it does not use tag ID in the URLs this leads to converting the first hyphen of the tag to the colon character. That's why we strip the hyphen character and that's why currently there is no way for using hyphens into tags.

I understand there are limitations at the moment, but these are not absolute...to be more precise, I have also tested ZOO and it allows punctuation marks in the tags without using IDs in the URL...
So it certainly is possible, although I honestly don't know how hard...but there is a ready-made solution that could be adopted or advised to solve whatever issue and offer this functionality...

K2 Team, will you please look into it again?

I am so disappointed by the tag extensions out there available for Joomla 2.5...this would be very welcome and I do prefer K2 very much over Zoo in most respects...

This is a quite big change and that's why we cannot add it to the 2.x series.

I see...

Is there some (loose or not) timeframe for the relase of K2 v3.0?