Can I buy a development card in the special building phase?
yes.

Taken from the  5-6 players rules pdf: http://www.catan.com/files/downloads/settlers_5-6_rv_rules_100107.pdf

emphasis mine:


  • Special Building Phase – Your opponents may build as outlined below. The
  Special Building Phase occurs just after the end of your turn (i.e.,
  between player turns). All the other players may participate in the
  Special Building Phase. Each player then takes his turn clockwise
  around the table, and is allowed to build anything he can create with
  his resources. 3 Note that no player is allowed to play Development
  Cards during the Special Building Phase. Also note that players are
  not allowed to do any trading with other players—nor are they allowed
  to use Maritime Trade—during this phase. They may only use the
  resources they have in their hands. For this reason, players are
  advised to trade as much, and as advantageously, as possible with the
  current player during his trading phase. They will then have the
  resources they need to build during the Special Building Phase. The
  Special Building Phase is an opportunity for all the other players
  (who are not currently taking their turn) to build roads, settlements,
  and cities, and/or to buy Development Cards. This phase allows each
  player to influence the game, even though it is not his turn! ...


This is also supported by the catan faq:


  Development Cards in General - If I have bought a Development Card
  during the special building phase right before my turn, may I play
  this card on my turn?
  
  Yes.