The acquisition may not clear anti-trust concerns.
Unless the regulators deem both companies / 8-bit mcus so hopeless in competition vs. 32-bit chips that even the combined entity will not gain much power over their customers.
edit: from that perspective, if the deal is stopped on anti-trust basis, it would be a great piece of news for both atmel / microchip investors; if the fed offered its blessing, well, great for atmel investors, and terrible for microchip investors.