In a document on the website of IBM, several details of the PowerPC 970 are made available. This processor uses, as the name already indicates, the PowerPC architecture and has been based on Power4, IBM’s hi-end processor.
Since Apple also uses the PowerPC architecture and the new processor also supports AltiVec, Apple probably will use the PowerPC 970.

With the introduction or the PowerPC 970, IBM has taken the PowerPC performance to new heights. With clock-speeds to 1.8 GHz, the PowerPC 970 is the fastest PowerPC yet introduced. But the 970 employs much more than frequency to answer the demands or high-performance computing customers. The 970′s multiple execution Units including an AltiVec compatible vector processor are fed by an 900-MHz processor interface bus, which can deliver rates up to 6.4 GBps.

Besides the above, the 970 is a 64-bit processor, with a 32-bits compatibility mode. This means that very little changes have to be made in OS X to support this new CPU, and that later in the process OS X could become a native 64-bits OS.