IMHO, the answer is somewhere between August 1, 2011 and Never. The August 1, 2011 date is the 3 year anniversary of the merger; which would clear them of the Section 182 restrictions.
But I'm not sold on AT&T ever buying them outright because of the concentration of licenses in the S-Band that they would have. AT&T has gathered a large number of WCS licenses, IIRC, which could put them in a bit of a problem with the FCC. Note that Verizon and AT&T have been forced to divest various licenses because of owning too much of one market as they've been buying up land. If they were to buy both DARS licenses (XM and Sirius), it could be looked at more closely than the XM and Sirius merger was looked at, IMHO. So I'm not quite sold on it ever happening.
JohnnyIrish - the first ownership change was the XM and Sirius merger on August 1, 2008. That is why the restrictions to Liberty run through August 1, 2011 (the 3 year anniversary of the merger). Sirius XM is now under the Section 182 restriction... it is why Liberty has been limited to 49.9% ownership and the reason why Sirius won't allow anyone to become 5% filed owners - so that the two combined wouldn't constitute the second ownership change with 3 years.