This new hard cap is due to the guild system that's coming in the expansion.
UPDATE: Blizzard has increased the guild cap to 1,000 members.
Blizzard just announced that a single guild will be able to have a maximum of 600 members in Cataclysm. This new hard cap will go live with patch 4.0.1. The soft cap used to be 500 members will some allowances given, but the hard cap is necessary due to the new guild system that's coming in the expansion.
According to Lead Content Designer Mumper, the 600-member cap covers "more than 99.9% of all the active guilds in World of Warcraft." Guild that make up the remaining tiny fraction will be able to keep their guilds intact, but won't be able to add new member until they drop below the 600-member mark.
So, how many members were in your largest guild? Have you ever come close to 600 members?