Space Marines and Tau have been mentioned recently in the rumor mill, and this small bit mentions both of them as being in a box set together. That would be a very interesting box set, and this set has it pegged for a fall release.
Please remember that these are rumors. I wish there was more to this, but this is it.
via Steve the Warboss on Faeit 212
-Space Marines vs Tau
-Release in Autumn