AMD video cards that use the same GPU can usually enter a symbiosis with each other, this being the fabled CrossFireX configurations.
The folks at TechPowerUp discovered that the Radeon HD 7870 video boards based on the new Tahiti LE GPU can enter CrossFireX with HD 7900 cards as well.
Some expected this, because the Tahiti LE-based HD 7870 cards are basically toned-down HD 7950 boards (full Tahiti).
Others were surprised because they had expected the memory amount, the interface and the lack of parity to prevent this possibility.
The test managed to pair Club 3D HD 7870 JokerCard with a Radeon HD 7950 and HD 7970 (reference). There is no reason for the other cards to not possess the same ability.
Oddly enough, the Tahiti LE cannot pair with Pitcairn, the GPU utilized by the common HD 7870 and HD 7850.