Full Story: 1up.comAccording to a "very credible source," there have been two important changes to the Achievements rules since that Gamerscore blog post in 2007: First, the new maximum points allowed for any retail game (including subsequent DLC) is 1,750 points, spread across no more than 80 Achievements. That puts those aforementioned titles that broke the 1,250 rule in the clear, although it still leaves Valve's The Orange Box unaccounted for with its 99 Achievements. We're assuming The Orange Box received special ex-post facto clemency, because it's awesome.
The second rule is that every game can add a maximum of 250 new points and 10 Achievements through DLC every yearly quarter (but only until reaching the 1,750 point cap). And evidently this rule is also cumulative, so that theoretically, a developer could release a new game with 1,000 points, not add any new achievements for the next three quarters, and then release a massive 750-point DLC pack in the fourth quarter. The anonymous source explained this rule is meant to discourage developers from releasing a ton of content and all 750 points immediately after a game's release, and instead doll it out slowly over time.