Some of the thinking behind Lightroom 3's grain effect is revealed in this 2008 article by Eric Chan, one of the top guys in Adobe's Camera Raw team Ten Tips for Better Prints

The digital printing process requires resampling an image from its original resolution to a printer's own resolution. This resampling process can produce visible artifacts such as blocky edges and areas that appear unnaturally smooth and flat. These artifacts become more noticeable as the image is enlarged, especially with really big blowups. Adding noise breaks up these artifacts and provides the illusion of a more natural, detailed appearance, even though no real detail has been added.

In other words, don't just think of grain as a way to simulate HP5 or TMax. Speaking of which - and you know how much I love my presets - look out towards of next week for some b&w presets I'll be unleashing on an unsuspecting world. Code name Gekko.