As long as the cards are screwed in good, I don't see a problem with the video cards coming loose.
Occasional bump down the road won't effect the only moving parts on it (fans) all that much. Assuming you are only using SSD drives. (not Hybrid!).
As far as latency. Note you will be doing battle with everyone else on that particular WiFi access point for the bandwidth and just the normal distance thing.
I would expect your keyboard/mouse latency to be non-issue compared to the 150-200---400ms you're going to see out of some of the Access point areas.
http://www.stealth.com/portables.htm
On a cheaper note: If can find them, I have a scientist that travels all over the world with his Shuttle PC. It has twin nVidia video cards and 4 SSD drives. He used to have normal hard drives in it, but they were failing and he wanted something more rugged. That PC probably has over 400,000 miles on it, including icebreaker time.