A CD that's been shredded in a straight-cut shredder can be reassembled, and is by the FBI, says Frank W. Abagnale, whose qualifications include "lectur[ing] . . . extensively at the FBI Academy and for the field offices of the Federal Bureau of Investigation." http://www.abagnale.com/index2.asp
cited at http://www.wnyc.org/shows/bl/episodes/2008/04/24
for segment "It Takes a Thief", as accessed Apr. 26, 2008, originally broadcast 2 days earlier (24th). Within the total playing time of 36:12, the specific discussion is from time-point 9:24, especially 9:39, to 11:42, as played on RealPlayer v10.5 on WinXP.
He said he can reassemble paper from a straight, ribbon, or strip shredder in an hour, completely, and from a crosscut, crisscross, or diamond shredder in 8 hours. Based on that, I assume CDs cut in a crosscut shredder can also be reassembled, albeit more slowly and less reliably.
The microcut or security shredder essentially silently pulverizes paper and CDs into "sand", he says; and he claims he can't reassemble from that (maybe I haven't seen the size of a piece from one of those).
I wonder how much integrity the CD's recovered data has for forensic purposes, depending on the kind of data. Sometimes recoverers may get everything they need but pits are small and some must get lost in the shredding.