It still remains unclear what the original source was, but Bleeding Cool is now reportng that they've been contacted by William Peter Blatty, who let them know that said news was "utterly false." According to Blatty, it would be impossible for Morgan Creek to produce the show as the rights still belong to Blatty himself.
On a side note, Blatty showed interest in developing a mini-series based on the film back in 2009 which was to be written and directed by himself, although it's now 2012 and nothing has solidified as of yet, Blatty has gone on record as saying, "Friedkin and I have a new miniseries remake script that we hope will one day be picked up."
So it certainly look like we may still see an Exorcist TV show in the near future.
As for the "other" mini-series, who knows... if its true, maybe the beginning of a rights dispute are in the making.
Source: Bleeding Cool