PC Games Hardware writes the ATI RV770 actually has 900 stream processors, but that only 800 of them are currently activated:
The RV770 is supposed to have 900 stream processors of which only 800 are currently activated . The RV770 would theoretically improve its shader performance by 12.5 percent with the additional 100 stream processors.
According to Hardware-Infos the additional ALUs are only used due to redundancy reasons. Technically the activation of the 100 shaders would not provide any advantages in practice except for GPGPU computing. It is nothing new that AMD installs more stream processors than actually needed since chip defects can be compensated very fast. The chips on which all stream processors are working can also be sold as upgraded models.