I haven't forgotten my WoW achievements project; just haven't gotten over the hump of building a general purpose reporting webapp. Spurred on by a request from my old guildie Alisha, here's some quick numbers on how folks are doing with Ulduar/25 completion as of June 26. 16% of people have completed the first section of Ulduar/25. 3% have completed the whole instance on easy mode. Hard mode achievements are < 1% of the population, so not repeating here. Percentages are based on a sample of 90,000 characters who reached level 80 before April 2009; you'll see below 44% of them have finished Naxx/25.
16% Heroic: The Siege of Ulduar
13% Heroic: The Antechamber of Ulduar
7% Heroic: The Keepers of Ulduar
3% Heroic: The Descent into Madness
71% Heroic: Besting the Black Dragonflight
44% Heroic: The Fall of Naxxramas
29% Heroic: The Spellweaver's Downfall