Most people connect this with Dundee’s line in Crocodile Dundee (1986): “That’s not a knife; that’s a knife”, where he holds up a large bush knife to an attacker’s (mugger’s?) comparatively petite switchblade. Notice how we’ve changed the pronouns to be more contrastive, from “that” and “that” in the original to “that” and “this”. “This” brings the speaker and his X closer to the listener/reader. Even people intending to reference the movie directly remember it this way, perhaps because there’s something more natural about a that/this pairing in a contrastive statement than a that/that or this/this pairing.
Variants on X are many. My favorites in snowclone.pl’s top results include race, gun, helicopter, sandwich, hard drive, mullet, blog, and miniskirt. X is usually a tangible object: we want to be able to compare that X to this X and see that the latter is more impressive.
[Taken from the list at the “Unblogged Snowclones” on Language Log.]