
According to Xbox360Achievements.org, there is an upcoming update to Microsoft's Achievement Police for Xbox Live Arcade games! Each time there has been an update, they have successfully and legitimately scooped the news, so time will tell if this remains a rumour or not. Currently, an XBLA game can have a maximum of 20 achievements that total 200 gamerscore, and each DLC pack can have a maximum of 5 achievements that total 50 gamerscore up to a maximum of 3 DLC packs. (1 pack per quarter.) That's 35 total possible achievements for a maximum of 350 gamerscore.
Under the new rules, XBLA games will have a maximum of 30 achievements that total 400 gamerscore, and each DLC pack can have a maximum of 8 achievements that total 100 gamerscore up to a maximum of 4 DLC packs. (1 pack per quarter.) That's 62 possible achievements for a maximum of 800 gamerscore.
This comes on the heels of the recent announcement that retail 360 games will be capping out at 2,000 gamerscore.
The new rules come into effect between April 1 and May 31, and becomes mandatory for all games published after June 1.
Update: No longer a rumour, here's the word right from the horse's mouth. (Thanks @[member='prowler'])