John Malone, the largest shareholder in AT&T, resigned from its board Tuesday, a move that appeared to be the culmination of a rift between him and the company’s chairman, C. Michael Armstrong. In a letter of resignation, Malone expressed displeasure over his exclusion from discussions with Comcast over that company’s offer to acquire AT&T’s cable business, a business essentially created out of the cable company that Malone founded, Tele-Communications Inc. It is now called AT&T Broadband. Analysts speculated that Malone may now make a bid for AT&T Broadband himself, probably partnered with another non-cable media company, most likely Microsoft. Meanwhile, Wednesday’s Los Angeles Times reported that Charter Communications, controlled by Microsoft co-founder Paul Allen, is also likely to make a bid for AT&T Broadband if it can find the right strategic partner.