Pimco was using basket futures swaps collateralized with TIPS to replicate the DJ-AIG index. The IRS told them that the derivatives they were using to replicate the index were not kosher per the tax code and therefore theey had to stop doing what they were doing or they would lose the special tax status that mutual funds have. I believe Pimco appealed this stupid ruling. In the meantime, they started rolling the portfolio over into structured notes, which is what QRAAX uses. Structured notes are basically a bond issued by a trust that holds whatever it is you want exposure to (in this case, TIPS plus basket commodity swaps). The bonds then pay interest equal to the return of the underlying investments.
Its a shell game and the switch to the notes doesn't change the economics except to add a bit more expense (like 10 BP). Not a big deal, IMO, but I would love to find something cheaper.