The Lead Content Designer, Daelo, announced beta testers will be ready to test Cataclysm raid encounters on Sunday, October 17th. Coincidentally, a few hours ago a new patch was deployed on beta servers (62MB): the beta client build

Two raid encounters will be open for testing partially: Blackwing Descent and The Bastion of Twilight (high in the sky at Twilight Highlands).

Players with a flying mount can reach the throne of Nefarian high in the Blackrock Mountain to reach the portal.

Blizzard Quote:

We’re going to start testing a few raid encounters on all Beta servers this Sunday, October 17 at 13:00 PDT. Two encounters will be tested:

Omnitron Defense System in Blackwing Descent
Halfus Wyrmbreaker in The Bastion of Twilight

Blackwing Descent is a raid zone with an entrance located along the side of Blackrock Mountain.
The Bastion of Twilight’s entrance is located within Twilight Highlands.

Since these are the beta realms, this schedule is subject to change at a moment’s notice due to bugs and server issues. More raid tests will be occurring over the next few weeks.