According to the Wikipedia page on African Union, Haiti is currently a member observer of the AU and current pending full Associate Member status due at the AU summit in July 2012 (sic). Haiti is in the Caribbean. What interest does Haiti have in (associate) membership of the African Union, and what interest does the African Union have in an association with Haiti?
'[The AU's] constitutive act declares that it shall "invite and encourage the full participation of the African diaspora as an important part of our continent, in the building of the African Union".'
The idea of an "African nation" includes those who (through acts of conquest and slavery and emigration) are no longer living on the continent of Africa. Haiti is populated by a majority (95%) that are descended from African immigrants, mostly African slaves (although most Haitians have a complex genealogy that includes European and Caribbean native DNA)
Thus Haiti is African in the same way that New Zealand is European. It is up to the AU to decide if that qualifies them for membership.