The January 2009 issue of Europe Echecs mentioned several rumors concerning the Grand Prix that I hadn't heard before, all apparently emanating from Dresden -- that Doha was cancelled partly because of a 'divergence of funds' by FIDE officials; that when the Grand Prix was announced at the beginning of 2008, only two of the six contracts had been signed; and that the next (fourth) event will also be held at Elista.
It's becoming impossible to follow what is happening with the event. The official site (grandprix.fide.com) still lists Doha and Montreux as venues for the third and fourth events, and has the same player list announced at the beginning of the series.
As for the four players replacing those who have bowed out, it's unclear what their status is. In mid-December, Fide.com posted Kirsan Ilyumzhinov at Elista FIDE Grand Prix (subtitled 'Press conference of the FIDE President'). When asked, 'Which principle was used when the withdrawn players were replaced by the new ones?', Ilyumzhinov gave an answer that I've been unable to decipher.
For my previous posts on the Grand Prix, see
What a mess.