Re: could you make a mini reactor out of a piece of Americium

There are (unfortunately) two answers to your question.  Yes, it can be
done, no, not on a small scale.  It is certainly possible to construct a
power source with Americium that would last 20-25 years.  It would weigh
over 200 pounds and it would have a surface temperature of over 150F.  It
would be called a radioisotope thermoelectric generator (RTG for short). 
It would also be illegal to own one as a civilian.  Important safety tip:
never dismantle the Am source inside of smoke detectors (which I'm guessing
was going to be your source).  It is an unnecessary hazard to you and
usually creates a serious problem at the landfill where it ends up.  

Since that wasn't all that helpful in you quest for a small robot power
source, I will offer some ideas.  If you want your robot to be remote
controlled, you could buy one or two of the mini-RC cars that are popular
now (I have several).  Having done some extensive (expensive) testing of my
own, I would use the Tomy Microsizers (Bit Char-G in Japan).  The batteries
are better and the gears are more precise.  You have numerous motors and
gearing options available depending on your torque or speed needs.  You
could also take the battery out and solder leads to the power posts and run
it with cables via a DC power supply.  You would still need the remote for
steering.  This would only be good for a really small robot (< about 4 to
6oz. with the low speed motor and the high torque gears).  Just a quick
note, the 27MHz and 49MHz are the only FCC approved frequencies for RC use
in the US.  There are more frequencies available in Japan.  
On the web search for micro sizer and you will be overwhelmed.  

If this isn't powerful enough, a hobby shop or possibly a large toy store
might have some still small, but not really small RC cars for you to
cannibalize in a similar fashion, just without the gear changing abilities.
 If you go with one of these, the DC power lines become more attractive as
large Ni-Cd batteries are (as you know) heavy.  

If you do have a top notch hobby shop near you, you may be surprised by
what they have.  I did a quick search on Yahoo Yellow Pages with your city
name and just "hobby" in the search and got about 15 decent looking hits
within 20 miles of your town.  Call around!  Just expect to pay a premium
for the diversity available at a shop.  

Good luck and good robot building, 

Scott Kniffin
Sr. Engineer
Radiation Effects and Analysis Group
NASA Goddard Space Flight Center