ICE is an acronym for "Intrusion Counter-measure Electronics" first coined by Tom Maddox, and later used by William Gibson in the Burning Chrome and Johnny Mnemonic short stories, and in the novels Neuromancer, Count Zero and Mona Lisa Overdrive.
"Normal" usage of the word "ïce", usually with cold drinks, features in most of William Gibson's novels.
In Spook Country there is a seems to be another use and explanation of a different ICE acronym:
chapter 32 Going Away
page187
Tito handed him the leather-cased badge Eleggua had taken from the pursuer's belt.He fipped it open. "Ice," he said and pocketed it.
chapter 50 Whispering Gallery
page 228
"They're Cuban-Chinese and they're probably all illegals"."Can't you get ICE to roll them up for you?"
chapter 54 Ice
page 242
"Why did you lift that Immigration and Customs Enforcement badge, Tito?" the old man asked.ICE. Tito remembered Garreth saying "ice," when he'd handed him the thing.
ICE refers to Immigration and Customs Enforcement in the Department of Homeland Security in the United States.