In an article entitled "Talking about Minifigures" (that starts on page 62), there's an interview with Matthew Ashton, Senior Creative Director of LEGO Playthemes and IPs (Intellectual Properties), who's the Lead Designer for the Collectible Minifig Series.
In page 65, the author of the article, quoting Ashton, wrote:
... And yes the designs are well underway! We are currently working on Series 6, 7 and 8 and all characters are locked on those collections. Series 6 is completed from a design perspective and we are cracking on the elements and decos for Series 7 as we speak. So to illustrate how far ahead we are, we are working on Series 7 as Series 3 is just hitting retail shelves (January 2011)...
So it looks like we'll see lots more series of Collectible Minifigures.