Is the Sata 6.0 discs backward compatable to Sata 3.0?

I am updating a friends computer.  I will be purchasing a new WD drive, but need to know if a Sata 6.0 will work with Sata  3.0 controller?  Or should I buy a Sata 3.0 drive?

Thank you

My research, so far, determines that most Sata 6.0g drives may not be backward compatible.

It appears, per the information provided at different sites, that only the Caviar Black with the “Dual processor technology” (Such as WD1002FAEX) is backward compatible with Sata 3.0g.

So to answer my own question, I will buy the dual processor version.

If anyone has any further information, please respond.

Thank you