Asus 100k PCBs per day is enough to earn some profit for a fabless semi firm. Now as the XILINX ZYNQ system architect in platform planning, this iPad2 teardown and A5 analysis listed is interesting in that ZYNQ has a dual ARM A9 + 128 bit NEON vector processing + Programmable Logic and a wealth of validated high performance soft IP cores to drop into the PL all of which is 28nm TSMC. XILINX will let the world know when that innovation platform is ready for your designs.
Apple designed my SigmaTel team's AV SOC out of iPod Shuffle Gen1 when Samsung offered a pairing discount for flash. It was hard to compete with that on price. The Asus fab produced over 100k PCBs with out part and embedded software though, before it was done.
It seems Apple already gives TSMC a lot of business, even if it's not the core processor or leading edge. Also TSMC does not manufacture its own tablet competitor, of course it's harder to leverage.
On the other hand, TSMC and GlobalFoundries both make Snapdragon for Qualcomm, which is used to power the HTC phones competing with Iphone. So I don't see why ultimately Apple won't use both Samsung and TSMC. Ultimately it can impose capacity burden on competing products at least indirectly.