My pen pointer stopped working on my Bamboo Fun

You probably dropped the pen - there is a very fragile ferrite core in the tip and when it breaks it behaves just as you describe.  I have posted a description of the repair procedure (adapted from a description for other pens that I had found on the web) on my blog: http://wishidknownthat.blogspot.com/2009/04/fixing-wacom-bamboo-pen.html

It takes a bit of patience and skill but you can do it.
I am pretty sure that this kind of "surgery" would void any warranty - on the other hand, if this is indeed your problem they will say you abused the pen (by dropping it) and charge you for the new one anyway.