I'm retrofitting a toy lantern for my daughter.  It has 2 AA batteries 
and a cheap incandescent flashlight bulb in it, which of course eats 
through batteries.  I want to put a white LED in it for better 
efficiency, and have a timeout function so if she leaves it on (which 
she will) it shuts itself off after a few minutes.

There's many ways to do this of course.  I could use a PIC to perform 
the timeout, but since I need a boost converter to power the LED anyway, 
I want to use the boost to get maximum operating time from the batteries 
and continue operating when the battery voltage falls well below 2V, 
which the boost converter will do, but he PIC won't (2V minimum for 
PIC).  Powering the PIC off the boost would be tricky since the PIC 
would have to shut down it's own power supply!

Then I started thinking maybe I should try to simplify and use a simple 
RC circuit to shut down the converter.  I'm going to use a MAX1674, 
which has a shutdown input (/SHDN) which shuts off the converter when 
/SHDN < 20% of Vout.  I figure if I put a capacitor from Vout to /SHDN, 
and a resistor from /SHDN to ground, that will do it.

I haven't tried this yet because I have the MAX1674 in a SMD package and 
breadboarding will take some time.  So for right now I'm just trying to 
come up with ideas.

Do you think this will work?  Any better ideas?

I got it.  A 12F PIC, two resistors, and two capacitors gives me ideal 
results.