Eric, thanks for your summary. I think this thread has shown that, with possible exception of some subtle scaling issues, the utility appears to work as designed to do. As Doyle has noted, turning off CM in the printer driver successfully may still be an issue in some instances, but I was able to implement his recommended patch to Canon's special casing AppColorMatchingInfo.xml file and it works as it's supposed to in my case.
All that said, there still seems to be a serious CM problem when actually trying to use a properly built profile, at least for some of us PSCS5/OS10.6 users. Because there is more than one method of invoking CM even for a single Adobe app, like target printing, we should be seeing matching print output from anyone of these chosen paths, but I'm not getting that result, at least not for my Canon 8100. For that printer, I appear to have three pathways available (not including the 16 bit plugin pathway). I can 1) let Photoshop manage color and select profile and rendering intent on the preview page making sure CM in driver is off, 2) choose printer manages color and use colorsync, or 3) choose printer manages color and select "vendor matching" in the canon driver. Bottom line: I can't make any of these paths work right when using PSCS5. I spent a few hours trying again last night and found once again that I have to drop back to printing from ID to get output that matches the PS or ID softproof view with reasonable accuracy. The ugly printed output I'm seeing from PSCS5 is not due to double profiling. Rather, the output looks like the source tag is being ignored. In my testing the image was tagged with prophotoRGB but the output looks something like what you'd see if you stupidly reassigned an sRGB or monitor profile to the image and then convert to the destination profile.
I'm going to compare "printer manages color" to "photoshop manages color" on an Epson 3880 today. Perhaps my problem is an isolated canon driver issue.