A MacBidouille's reader notes that the following was posted on Apple's Darwin x86 forums:

"Customers with existing PPC G5's will have the option of a PCI-X based add-on card with a native Intel follow-on to the P4 "D" chip and of-course the Transitive/"Rosetta" bundle. Any other PPC machine bellow the G5 (i.e. G4, G3 machines) are not included on the current roadmap inclusion. Third-party providers have shown interest is providing a solution for down-level Macs."

We still don't know if Apple itself will provide such a card and if this card will allow both of the processors to work at the same time.