Smallest/cheapest possible Python platform?

[Roy Smith]

What's the smallest/cheapest/lowest-power hardware platform I can run 
Python on today?  I'm looking for something to use as a hardware 
controller in a battery-powered device and want to avoid writing in C 
for this project.

Performance requirements are minimal.  I need to monitor a few switches, 
control a couple of LEDs and relays, and keep time over about a 30 
minute period to 1/10th second accuracy.  Nice-to-have (but not 
essential) would be a speech synthesizer with a vocabulary of maybe 50 
words.

The Rasberry Pi certainly looks attractive, but isn't quite available 
today.  Can you run Python on an Arduino?  Things like 
http://www.embeddedarm.com/products/board-detail.php?product=TS-7250 are 
more than I need, and the $129 price probably busts my budget.