KJP and puffin,

I believe the research shows that kids learn better when grouped by ability -
1) http://www.casenex.com/casenet/pages/virtualLibrary/gridlock/groupmyths.html
2) http://www.gifted.uconn.edu/nrcgt/reports/rbdm9204/rbdm9204.pdf

Addressing a different aspect of this discussion thread: Because the OP's article mentions "tracking", this may be a good time to re-iterate some definitions:

Tracking. A rather permanent group consisting of age-peers moving together through the grade levels. Pupils are generally advanced in all subjects. Commonly receiving curriculum instruction one grade level ahead of gen-ed age-peers. For many HG+ pupils, this is not enough curriculum advancement for them to learn something new each day, remain challenged, and engaged/achieving. When kids get on the "track" they typically do not leave; Similarly, new kids may have a difficult time getting on the track, as a "track" is generally considered closed. A magnet-school-within-a-school may be a form of tracking: a student is either in it, or not.

Cluster grouping. Originally called flexible cluster grouping to distinguish it from tracking. May include pupils from different grade-levels. Pupils may be advanced in one or more subjects. Students may have single-subject acceleration (SSA) of one or more years. The ideal may be flexible cluster grouping by readiness and ability, regardless of age or grade level, therefore combining children of various ages, classrooms, and grade levels.
These links provide more information on flexible cluster grouping by readiness and ability:
http://www.casenex.com/casenet/pages/virtualLibrary/gridlock/groupmyths.html,
http://www.gifted.uconn.edu/nrcgt/reports/rbdm9204/rbdm9204.pdf
Unfortunately, the buzzword "cluster grouping" may be used (mis-used) to mean one or more gifted kids within a particular classroom, not necessarily being taught at a higher level but rather being treated as somewhat auto-didactic (often due to schools buying into the myth that because they are gifted, they will be fine on their own).