Director Danny Boyle is talking about directing the third installment,
28 Months Later, which he has considered doing for some years now. Boyle directed the 2002 original and Juan Carlos Fresnadillo took the helm on 28 Weeks Later in 2007, with Boyle as an uncredited assistant director. Boyle seems to be in the mood for horror: he is set to direct a stage adaptation of Mary Shelley's Frankenstein. Bottom line on why he'd direct a horror sequel: he owes two films on his three-picture contract, signed after Slumdog Millionaire won the Oscar, with Pathe and Fox Searchlight (where he made 28 Days Later, Millions and Sunshine). The upcoming 127 Hours is the first film in the deal.
(UPDATE: title is to be determined and we shouldn't hold our breath for the film - no script currently exists for it).