Sankoff & Mazzie (1991) investigate the extent to which prenominal wanpela 'one' and dispela 'this' have become grammaticalized as articles. While they find little evidence, Smith (2002: 149–150) demonstrates phonological reduction of dispela in his corpus which may indicate grammaticalization is occurring. He also points out (p.148) that the use of postnominal ia (or ya) is becoming obligatory to mark known human referents, at least in his data.
Source: nd