I know I'm replying to myself, but for others who might be interested, Safire chimes in on the origin of "spider hole" here:

http://www.sltrib.com/2003/Dec/12162003/commenta/120316.asp

The relevant snippet:

[safire]Another useful bit of information is the origin of "spider hole," a phrase used by Lt. Gen. Ricardo Sanchez to describe the dugout hiding place in which the fugitive Saddam was cowering.
This is Army lingo from the Vietnam era. The Vietcong guerrillas dug "Cu Chi tunnels" often connected to what the GI's called "spider holes" -- space dug deep enough for the placement of a clay pot large enough to hold a crouching man, covered by a wooden plank and concealed with leaves. When a U.S. patrol passed, the Vietcong would spring out, shooting. But the hole had its dangers; if the pot broke or cracked, the guerrilla could be attacked by poisonous spiders or snakes. Hence, "spider hole."[/safire]

Cardhouse (http://www.cardhouse.com)doesn't buy his logic:

[cardhouse]That seems like an awfully long road to travel to get to the term "spider hole." Why not "snake hole"? Why would American forces name things from the perspective of a clay pot-hiding VC? "Hey, I found one of those dead clay pot VCs ... looks like he died from a bunch of spider and/or snake bites! I thus call this hole and all subsequent holes formed in the same fashion ... a .... a spider hole!" It couldn't have possibly derived from the fact that spiders actually dig holes in the ground or that VC tunnels were sometimes "guarded" by a ton of spiders? [/cardhouse]