I just thought I'd share some anecdotal evidence about my shimagnolo experience. I searched this forum for this kind of stuff when I was building my bike, and it was really helpful reading people's actual experiences putting these combinations together, rather than simply a general statement about "Shimano + Campagnolo = Boom" or something to that effect. I figure the more the better, so here goes.
I have the 10 speed 2007 Veloce brifters going to a 9 speed Shimano Ultegra cassette through a (nominally) 10 speed Ultegra derailleur. To account for the different cable pull lengths, sprocket pitches, and rear derailleur pull ratios, I used the "Hubbub" modification I found on this incredibly useful site:
Not only does it offer possible shimagnolo combinations, but it describes in plain terms how shifters and derailleurs interact, which helped me understand how and why certain combinations did or didn't work.
Anyway, so far, shifting is right on target. As in, I visually checked the alignment of the rear pulleys with the cogs at each click of the shifters, and they line up right on the middle of each cog. This campy10-shimano9 combo is mentioned to work on the CTC site and so I am simply affirming that it works.
Also, I think it would help to describe what DIDN'T work. Without the Hubbub modification, we would expect the campy shifters to overshift. I attest that this is true. Starting from the highest gear (let's call this cog 1), I adjusted the indexing so that when I was on cog 2, the chain just barely rubbed cog 3. Indexing between the highest gear cog 1 and the second highest gear cog 2 worked fine, but when I clicked the veloce shifter once more, the chain jumped to cog 4, skipping cog 3. Shifting through the remainder of the gears was okay. However, because cog 3 was skipped, going all the way to my lowest gear required only 7 clicks, rather than 8. Essentially, I had lost one gear on my 9 speed drivetrain this way.
Now, before you think I am jumping to conclusions with this example, I will concede that my adjusting strategy was not optimized for a campy-shimano combo. Although adjusting the indexing so that the chain barely rubs the 3rd cog is standard practice for full shimano or full campy drivetrains, by using the same technique on my shimagnolo drivetrain, I was putting the mismatched indexing to the extreme, which is what caused the skipped gear. There are other threads on here that say you can calibrate the indexing to be "centered" in the middle of your cassette (rather than calibrating at the high gear end), and essentially using the high and low stops on the derailleur for indexing at the high and low cogs. I have no evidence on whether or not this works, but I can only guess that your drivetrain will be more finicky than a perfectly matched setup.
So there you go--with a little modification, campy 10 speed to shimano 9 speed works! No need for the jtek shiftmate. Although I have no qualms about getting a shiftmate, but it's an extra $30 out of the pocket for me.