Arduinos are great fun, and in theory could probably do a great job of
> replacing analogue components in older automobiles. Keep in mind, however,
> that they are consumer-grade electronic devices, and depending on the
> conditions that your particular automobile might undergo, they may or may
> not be robust enough to last very long. YMMV.
>

I would not use an actual Arduino circuit board except for initial testing;
they are big, relatively expensive, and have components such as
electrolytic capacitors that wouldn't be good choices in the harsh
environment of an engine bay.  The Atmega328 microcontroller itself, the
heart of an Arduino microcontroller, is quite robust.  Even the cheapest
version is rated -40C to 85C and for $0.15 more you get one that will go up
to 105C.  If your taillight area is getting above 105 degrees celsius,
you've got bigger problems than idle stabilization.  The final circuitry
would be potted in epoxy resin to keep out water, chemicals, and to provide
protection from vibration.  The main issue for durability is to provide
protection from voltage noise and spikes coming from starters,
malfunctioning voltage regulators, and dufus Samaritans that hook up
batteries wrong when jump starting.  That said, if it does fail, it won't
leave you stranded.

For others who are interested, to work with Arduinos you'll need to develop
> an understanding of two separate fields of study: programming and
> electronic engineering. That may seem daunting, but keep in mind that there
> is a huge and very supportive do-it-yourself community surrounding Arduinos.
>

The programming is pretty trivial as long as long as you have a clear
picture of what the input signals look like and how they should affect the
output to the idle stabiliiztion valve.  As far as the electronics go, I
belong to DorkbotPDX(motto: People Doing Strange Things with Electricity)
and there are several people well versed in all things electronical that
would be an excellent resource.

Brett
Portland, OR
"Albert" 82 Riviera