The sudden news is in sharp contrast to the years of speculation surrounding any possible intention of Emirates to fly to Liberia (the latest speculation is that Emirates is still serious about serving Liberia, possibly sometime in 2013). There was barely a week between the Liberian Airport Authorities announced the agreement that BA officially announced the commencement.
This is the third intercontinental airline to start new service to RIA since 2010, when Delta finally began its Atlanta-Accra-Monrovia flights (earlier this year, the routing was changed to New York JFK-Accra-Monrovia). In 2011, Air France extended its Paris-Conakry service to both Freetown and Monrovia; AF still flies twice-weekly to RIA. For many years previous to this, only Sabena, which collapsed and was replaced by SN Brussels, which was renamed Brussels Airlines, had the only flights outside Africa to and from Liberia.
However, none of these routings are dedicated to Liberia alone: all stop in another African capital on their way to or from RIA. This seems at least partially due to security concerns around refueling and crew way-stationing, along with considerations of whether little Liberia has the market to fill a 250-seat widebody jet nearly once per day. Emirates, if it does come, will likely include service to Freetown or Conakry as part of its flights to Liberia.
Its not exactly clear whether British Airways, or its predecessor British Overseas Airways Corporation (BOAC), ever flew to Robertsfield. BOAC's rival British Caledonian, which it swallowed in the same manner as it now is merging with BMI, flew from London to Monrovia for years, sometimes via Banjul, Gambia, or Freetown. Its not clear at this point if BA will sell tickets for the ROB-Freetown portion of its routings; currently ASKY and Royal Air Maroc serve this route a few times a week, and additional flights will offer some welcome competition to the capital next-door.