<p>What serial communication bus would you use in a automotive embedded system if your main selection criteria were:  </p>

<ul>
<li>Electrically robust  </li>
<li>Slow speed (32 kb/s)  </li>
<li>Easy to program  </li>
<li>Easy to interface with microcontrollers  </li>
</ul>

<p>It won't be transferring much data, but it will need to be transferred periodically at high speed (100 - 500 Hz). I find that RS-232 is just not reliable enough if you have noise in your ground line. CAN-bus seems interesting, but I have no experience with it. </p>

<p>We are currently interested in a combination of AVR AT90CAN128 microcontrollers.</p>

For all out reliability you can't go past CANbus (but then that was sort of implied by your choice of processor?)
Depending on what you want to interface to, this can be very simple - the base level protocol is quite straight forward. But if you want to talk to any other non-proprietary devices you'll have to implement the higher protocols (eg. CANopen).
But if CANbus isn't appropriate, then I would recommend RS485 or RS422 (depending on your topology). If all you're trying to achieve is point to point communications it's identical to RS232. (you'll have to put your own protocol layer on top if you're trying to support multidrop comms though.)