The story of AT&T's cable television unit begins a new chapter this evening, when bids are due from competing suitors for the unit, the nation's largest cable operation. AOL Time Warner, Comcast and Cox Communications, which are each already among the nation's biggest cable companies, are all expected to submit formal bids. Microsoft, meanwhile, is expected to propose acquiring a minority stake, as it looks toward winning a captive market for its own software products. AT&T, which has about 14.4 million customers, could in the end decide not to forge a deal with any of the suitors but to let the unit remain independent, as originally planned -- although such a choice is considered unlikely. Last year, AT&T announced plans to split itself into three companies with four separate stocks. The cable operation, which was assembled through AT&T's acquisitions of Tele-Communications Inc. and the MediaOne Group, was to be spun off in 2002.