I had a couple of small problems upgrading to 2.7.1.
Firstly, I am using automatic upgrade (which is usually magic) and it encountered an error with a function being previously defined. So I had to do a manual upgrade, which doesn’t appear anywhere near as professional.
After the manual upgrade, I had a problem with the appearance of this blog. Looking at the output HTML I discovered an error was drawn into the class attribute of the body element. The function join was being called with the wrong parameters. After some googling and grep searches for information about which functions are part of the PHP and Wordpress APIs, it appears that the functionality of the Wordpress function attribute_escape or the hook body_class doesn’t exist anymore.
To fix (though I imagine I’ll need to address this next time I upgrade) I edited function k2_body_class in k2/app/include/info.php so the attribute_join is called with the output of join as the argument, not vice versa as it was previously:
$c = attribute_escape( join( " ", apply_filters('body_class', $c) ) );
Perhaps an upgrade of K2 is a better idea?

It seems a fix was released at K2, but they have removed the escape_attributes function call.
Not sure if that is a good idea; so I’ll keep my own modifications. Though I did discover when sending my website through the html validator that I had this error in several places.
http://getk2.com/2009/02/wordpress-271-and-k2-rc7/
I’ve also removed Facebook connect from this blog, seems like a nice enough Facebook plugin, but the markup isn’t xhtml.