I tend to think if it's ambiguous it shouldn't be allowed.  I most definitely do not want to be consulting an operator precedence chart every time I try to read other people's code.

I personally think that shit like:

for(i=0;i<x.length;){ if(i+=++i+3){ x[i][--i]; continue; } i++; }

is starting to get really really bad... I wouldn't want to ever see 2000 lines of that shit.