See mailing list. The spec. should be clear on how the regions are becoming the containing block of the flow element fragments.
flowing in regionA and regionB with different sizes.
The width of the div will be 50% of the regionA width for the fragment that flows in regionA and 50% of the regionB width for the fragment that flows in regionB.
This is covered in http://dev.w3.org/csswg/css3-break/#varying-size-boxes now.