MacBidouille claims that IBM's production yields on PowerPC 970's greatly exceed those of Motorola's yields on the PPC 7457.
According to the French site, IBM is able to produce 105 PPC 970's from a 12" wafer with a 80% certification rate of 1.6-2GHz chips. Motorola, however, is reported to produce 98 G4 PPC's with only 1/5th the yield of IBM on (presumably a similar sized?) wafer.
These poor yields are claimed to be part of the delay of the PowerBooks, which are speculatively felt to utilize the 7457 processor from Motorola.