What is your cost basis in the Aetna shares?
This is what I think happens. Suppose your Aetna shares cost $200 each when you bought them. The payout is not taxable, but reduces your cost basis. So now your cost basis for those shares is $55, which transfers to CVS so your cost basis for each 0.8 CVS share is $55, or $68.75 per share. So the $145 payment is not taxable now, but the reduction in cost basis means you will have a larger capital gain whenever you sell the CVS shares.
[Edited: This is incorrect, see correct answer below.]
