Companies A, B and C share common ownership and the same health plan. Company A is being sold in a stock sale. Company B is being sold in an asset sale. Company C will continue to exist and remain as the sole sponsor and its employees the sole participant in the health plan as 1/1/10. Which Company's employees need COBRA notices?

It depends. You need to look at the definition of M&A Qualified Beneficiaries in Treas. reg. 54.4980B-9, Q/A-4(a) and (b).