I think your number is a bit high... Assuming that the reports that the cost to produce an iphone 4 are correct at $188, you're talking. 2 million * 188 = 376 million. Figure at most 500 million with shipping and overhead to send everyone a new iPhone 4. Also, the old ones wouldn't be completely useless, they would become refurbished phones assuming there is an easy fix that they could apply to them (antenna coating?), resurface the glass on the front and back and the metal band.
Even if your number is correct at 2 billion, its well within apples capability to do, look how much microsoft spent repairing defective xboxs.
I think the thread starter is underestimating apple, they've become one of the worlds largest companies, reports late last year showed that they had 34 billion in (free, spendable) cash alone. Apple could easily recall 2 million phones if they needed to.
Will they though? I highly doubt it.