I don't quite understand all the dynamics going on here but my sense was that part of the problem is one of scale, funding, and mandate. The African Union troops who have been on the ground since 2004, initially with less then 1000 troops and as of last october/november about 7000 strong, have constrains on how they can intervene given the international agreements that allowed for their presence in the first place. The African Centre for the Constructive Resolution of Disputes (ACCORD) has a good one page overview of the mission and its composition (pdf)

Wiki has an informative piece on the history of the AU troop presence although it is not fully up to date.

I don't understand the details of why the AU troops mandate is limited or why increased funding and greater mandate seems to require getting UN status (even if on the ground some of the same troops are present). However, the push at this point seems to be to get a UN peace-keeping force in place (see guardian article from January 26th).

Once you pass the intro section very focused for a US audience this PBS News program video on the African Union's Darfur Mission is quite informative on the bind of the AU troops as see by two humanitarian relief experts who returned from Darfur in Oct. 2005 and discuss the African Union's efforts to bring stability to the Darfur region.