Here's a source for keyboard and laptop covers that would go a long way to field harden a laptop.

http://www.protectcovers.com/

I just ordered a keyboard skin...I'm just as worried about the constant threat of inadvertent Starbucks spills as excavation grit, myself... 

The full laptop cover looks like a promising option for an on-site machine. Electrical tape can effectively seal up the other points of ingress (slots, jacks, connectors) not in field use.

Joe Roberts
W&M Anthropology