very difficult question. I very often found that whatever I leave behind is exactly what I desperately need afterwards... I have about as much equipment as you do but as I usually do not have any porters ;-) I'm very often leaving parts of it at home. For example, the 2nd camera body is nice to have but not absolutely necessary as long as the first one is in working condition.
I very well remember the day when I was changing a film in a bus near Cober Pedy (central Australia) and the driver had to brake suddenly. Of course, the film fell into the camera, damaging the shutter, which basically meant "no more pictures" for this trip...
Also, I recall many occasions where I hardly ever used a tripod, - but when I accidentally left it behind on a trip to Egypt, I was desperately looking a substitute in quite a few local shops.
If you happened to be there at the same time, I was the person improvising tripods with books and clothes on ancient walls... I now always carry at least a small pocket tripod in my bag, even when I only go out for a few hours' walk.
I always take my 50 mm (f 1.4) lens which is very useful in bad light conditions, mostly a Sigma tele zoom (f 2.8-3.5 80-200 mm) and sometimes a sigma 21-35 mm lens.
Very often I take much more equipment on a trip but decide before leaving the hotel what I'll presumably need for the day. This, of course, very much depends on a photographer's personal interests.
If you are not doing anything extreme (macro work or bird-watching....) you may well be able to do with one good zoom lens I suppose...
And one more thing: try to find out as much about your destination as possible before you leave. this way, you can judge whether you’ll need that extreme wide-angle lens or a monopod.
Don’t only look for information in the guide books: there are many relevant newsgroups, online communities and web sites providing you with up-to date information.
For those of us, who haven't made the switch to digital yet, bringing enough film can be critical. There's hardly anything more disappointing than running out of film somewhere in the Himalayas, - with the possible exception of actually finding a film shop only to discover lateron that the films have been stored under the wrong conditions!
I always bring plenty of film and run a test before I leave home.
When flying, don't put it in your main luggage though: the strong X-ray machines on airports are almost guaranteed to spoil it! Hand baggage should be safe, unless you are using highly sensitive film.
One should remember though that the effect of X-rays is additive, - so it's always worth trying to get a hand inspection of your film bag, - even though nowadays most airport staff will decline this.