I always assumed the answer was something to the effect of:
boolean siriEnabled() {
return (system.cpu.version >= 5.0);
}
Is anything else really needed? They don't want to support it on older models so you have to buy the new one. Conversely, if you really want the feature, buy the latest phone. Personally I find Siri an overhyped piece of junk. I have a 4S and I disabled it because it kept getting activated randomly and rarely understood my commands. Plus for the basic stuff like weather, I can just open the app. The anecdotal crap like "Will I need an umbrella today?" is just a dumb gimmick to me. But anyway, the fact is that the 4S is really an incremental improvement over the 4, and Siri is the one feature Apple can point to on the 4S as a differentiator, so they enforce that differentiation.