Xbox360Achievements.org reports, via an inside source, the base Gamerscore allotment will be raised from 200 to 400, with a max of 30 Achievements available per title instead of 20. Downloadable content will also receive more flexibility, doubling the potential Gamerscore procurement to 100, with up to eight Achievements allowed per quarter, with a max of four packs allowed over time.
The new rules will apparently go into effect between April 1 and May 31, with titles launching during that time having the option of going by either the old or new system. According to the site, all games after June 1 will be required to follow the new rules. We've contacted Microsoft for confirmation of the details.
Update: Major Nelson just posted a notice of the new policy.