It seems like a reasonable thing, but the reality is that laptop HDD are more than robust enough to work inside a vehicle. There are thousands of carputer users on mp3car.com and the collective experience there indicates that a SSD isn't necessary.
Personally, I've had both a carputer and a UMPC inside my truck for 5 years and 70,000 miles now and haven't experienced any hard drive failures, skips, misses, freezes, etc. Netbooks and UMPCs are designed to be toted around. Most of them have shock mounted hard drives already. Modern hard drives are used inside of iPods w/o a problem. I thought I'd need an SSD and worried about my HDD for years before feeling truly confident that it wouldn't fail.
Craig