As already stated in bug http://sourceforge.net/tracker/index.php?func=detail&aid=1798174&group_id=69596&atid=525124 I'm having this weird issue with driver 0.7.8-p3. I've checked with xev and everything works fine except sometimes some ButtonM events are not caught. This results in no clicks and even no scroll wheel action.
This seems to be happening only with the cursor device since stylus and eraser appear to work flawlessly.
I'll attach my xorg.conf but it is not a big surprise since I followed both the guide in the linuxwacom web site and the guide from Gentoo's wiki (http://gentoo-wiki.com/HOWTO_Wacom_Tablet).