I've had one in my car since July 2004 (Minnesota).
It's hooked up using the Denison ICE-Link.
The old gen 1 iPods (mono screen) would have a black screen for an hour or so on coldest mornings.
The color screen models don't suffer this problem. It's always readable.
The hard drive in any of them have never given me trouble with startup, sluggishness, etc.
In short, with the color models, I've had no problems will cold temps.
Speculation: constant trickle charge of the ICE-Link keep internals just warm enough to not freeze, stick, etc.