Namely, the following volumes have been canceled (to be resolicited 'at a later date'): Captain Carrot vol. 1, The Great Disaster vol. 1, Suicide Squad vol. 1, Secret Society of Super-Villains vol. 1, Jonah Hex vol. 2.
In their place, three volumes are being rush solicited: World's Finest Comics vol. 1, Sgt. Rock vol. 1, and Supergirl vol. 1; and two volumes will be retro-solicited in the October Previews: Brave & The Bold Batman Team-Ups vol. 2 & Justice League of America vol. 3.
I must admit that I was surprised when the now-canceled volumes were initially solicited. They all come from a period (the late 70s through the mid-90s) when for various reasons the reprint rights aren't clear cut. My understanding is that because of the work-for-hire contracts in use at DC at the time the reprint rights must be renegotiated or else the costs of reprinting are so high as to make them unprofitable. This is why we see so few DC comics from that time frame collected; it's doable, just time consuming; and unless DC has a current good relationship with the creators it can get sticky.
I had figured that DC had figured out a way around the legal quagmire; apparently DC did too, but with this batch of cancellations it would appear that they were wrong. Hopefully there's a solution forthcoming; I for one would love to have The Great Disaster and Secret Society volumes, and I know that there are many people who never read Suicide Squad the first time around who were looking forward to the Showcase Presents volume of that title.
Edit: Here's the relevant quote from Bob Greenberger:
DC pays a royalty based on a percentage of the cover price to writers, pencillers, and inkers to all material published prior to 1976 and after 1997. For the period in between, the vouchers that were in use called for a set reprint fee to be paid. In some cases, the amount of contractually obligated reprint fees makes the budget for a proposed collection unprofitable. In those cases, DC will either scrap the project or ask the talent involved to waive the reprint fee in lieu of the standard royalty arrangement. If the parties agree, then everyone benefits.
Edit 2 (Sept 10): According to Bob Wayne, the canceled volumes will "reappear sometime in the second half of 2008."