Minor modification/upgrade suggestion
 

The idea is really simple and should be just as simple to implement. Basically I would like an LED that is illuminated while I'm below an MPG threshold for DWL. This way I don't need to actually read the gauge, I can just have a light out of the corner of my eye (red) to indicate that I'm below my DWL threshold.

It would require just an LED and resistor connected to a pin, one more variable in the setup to modify easily, and just a snippet of comparison code. I'll see what I can come up with, though it may take me some time to figure out how to add a variable to ROM...

This sounds like a great idea!
I really like it.

I have a couple models that have a piezo on pin 10 (paulbs idea), was thinking that a buzzer might be useful feedback, but it has to compete with the other items on the wishlist and the limited space available.

Also, need to be able to "configure" the mpg to sound the alarm at via the menus, and turn it off, etc.

Cool, do you have any code snippets for that?

I put an example of some sound code on arduino.cc
Arduino Forum - pwm sound revisited, pitch and volume control

It uses the pwm hardware so the sound is done in the background, and has pitch and volume control. Wish I had another 12k of program space to play with :)

Very interesting, but I was more interested in how you tied the alarm into the MPGuino code to set it off - what you used to modify the value for comparison, etc. I'm sure I can figure it out, but why re-invent the wheel if you've already done it? ;)