Blizzard poster Tseric has posted a clarification on the official World of Warcraft forums regarding the meta socket system introduced in Burning Crusade. This is in response to a poster asking if players can get the bonuses from the gems they place inside the meta socket or just the meta socket bonus when requirements are fulfilled.

Here's what Tseric had to say:

The metagem bonus takes all of your equipped gems into account when filling the requirement. If the meta-gem requires two yellow and one red gem, those three gems must be socketed somewhere on your gear.