Many of the JSF books in the market dedicate a special chapter for intergating the two frameworks. As a matter of fact I have never used Struts before and directly started reading about JSF.
The integration is usually mentioned because Struts is the defacto framework in the industry. A lot of people have spent a lot of time developing their Struts application and by integrating the 2 the only part that really needs reworked is the view.
Not to mention that there is a draft proposal for Struts 2.0 codename Shale that suggests replacing Struts taglibs with JSF.
