Everything you could want to know about it (and more) is found here:
A guide to rear shifting | Cycling UK
The executive summary is that with post-2001 Campy shifters and pre-4700/5800/6800/7900 Shimano derailleurs, if you attach the shift cable in a peculiar way you can get the indexing to work pretty well with one fewer cogs on a Shimano cassette (i.e. 10-speed Campy shifters will work with a 9-speed cassette).
There are, as you mention, some reports of people "kind of, sort of" getting Campy shifters and derailleurs to work "straight up" with Shimano cassettes, but it relies on a little slop in the setup. This "slop" factor gets worse as you add more gears, so 8-speed will work better than 9-speed, etc.
BUT the two systems converge again at 11-speed because there just isn't room for as much difference in spacing.
The other theoretical possibility is using a post-2001 Campy shifter with a pre-2001 Campy rear derailleur to index a Shimano cassette with the same number of gears as the shifter is intended to operate. For instance, in theory, a 2002 9-speed Centaur shifter with a 1999 Racing T rear derailleur will index well with a 9-speed Shimano cassette. I'm actually trying to use this exact combination on one of my bikes right now, and the actual indexing does work as well as the theory suggests, I think, but I'm having a problem with the shifters themselves, possibly due to a worn out indexing gear. Consequently, I still believe this is possible but I haven't gotten it to work consistently enough to endorse it without caveat. I just bought a 10-speed shift body assembly (because new indexing gears are hard to come by these days), and I'm hoping that will work with a 10-speed Shimano cassette using my Racing T rear derailleur. However, my confidence level is such that I also bought a 2003 Chorus rear derailleur and have been shopping for another Campy-compatible wheelset.