Proxy is ubiquitous in Terran versus Protoss now, and its implications to the balance of the match up are debated by the community recently. This article is a follow up to my previous post on this topic.
Protoss often proxy their tech building early on for an attack, and Terran lose the game immediately when they are not prepared well. This post discusses the response toward the two mainstream proxy tech builds.
There is actually an interesting metagame for proxy now for TvP and TvT.