I know it uses the page title (it displays the page title without a problem.) The problem is that, when you insert the default code for the tweet button (the same code that is in the link you sent above), and then you click the button, it displays the “Site Title” twice.
When I click the Tweet button the result is something like this:
Hello world! | | My-site-titleMy-site-title http://www.mysite.com/blog/2012/01/hello-world/
Where “My-site-title” is what I entered as Site Title in the WP admin panel’s General Settings. The only “workaround” that I can find at present is to delete the Site Title from the WP general settings completely.
Again, this problem does not appear to be Theme-dependent and the same thing happens with both the default Twitter button and with the Sharebar Twitter button. The best hypothesis I can come up with is that it is either a problem with Wordpress 3.3.1 or a problem with the interaction between WP 3.3.1 and Twitter.