LONDON (Reuters) - The world's largest mobile phone firm Vodafone Group Plc (VOD.L) and U.S rival Cingular Wireless were poised on Friday for a head-to-head bid battle that could value struggling AT&T Wireless (NYSE:AWE - news) of the U.S. at around $35 billion. As the clock ticks to a 5 p.m. EST (2200 GMT) bid deadline, sources close to talks said Vodafone was putting the final touches on a bid, while Cingular was ready to fight back. But final bid strategies were expected to be left to the 11th-hour. "What happens in these auctions is that everybody is making decisions...