Lucario Guy, I understood it differently.
If I'm getting it right, it means this:
Beast Landorus: Intimidate / Sheer Force
Beast Tornadus: Regenerator / Defiant
Beast Thundurus: Volt Absorb / Defiant
Landorus: Sand Force / Sheer Force
Tornadus: Prankster / Defiant
Thundurus: Prankster / Defiant
On the plus side, this means that all of the DW abilities are now available. Special Landorus will be a fun, Nidoking-esque gimmick. Oh, and Defiant Tornadus is sure to be neat..
I think it's more like this:
Transferred BW1 Incarnation Tornadus: Prankster
Transferred BW1 Incarnation Thundurus: Prankster
Transferred BW1 Incarnation Landorus: Sand Force
Transferred BW1 Beast Tornadus: Regenerator
Transferred BW1 Beast Thundurus: Volt Absorb
Transferred BW1 Beast Landorus: Intimidate
Dream Radar-Captured Incarnation Tornadus: Defiant
Dream Radar-Captured Incarnation Thundurus: Defiant
Dream Radar-Captured Incarnation Landorus: Sheer Force
Dream Radar-Captured Beast Tornadus: Regenerator
Dream Radar-Captured Beast Thundurus: Volt Absorb
Dream Radar-Captured Beast Landorus: Intimidate
"When you change the form of the Tornadus/Thundurus/Landorus captured in the Dream Radar game by using the Reflecting Mirror, the Pokémon will have their Hidden Ability. That's Defiant for Tornadus & Thundurus and Sheer Force for Landorus. However, if you transfer the Pokémon from Black & White and use the item, they will transform into Sacred Beast Forme, but with the Sacred Beast Forme Pokémon having the same ability as if they were caught in Dream Radar."
The beast abilities are always the same.