<p>The last week on the ACM ICPC Mexico competition, I missed a "return 0" on a C++ program. For this reason we got punished with 20 minutes.</p>

<p>I had read that the standard does not oblige us to write it at the end of a main function. It is implicit, isn't it? How can I prove it? </p>

<p>We were using a Fedora system with a G++ compiler. </p>

You refer to the C++ Standard, chapter 3.6.1 paragraph 5:
A return statement in main has the
  effect of leaving the main function
  (destroying any objects with automatic
  storage duration) and calling
  exit with the return value as the
  argument. If control reaches the end
  of main without encountering a return
  statement, the effect is that of
  executing
```
return 0;
```
If you haven't got the Standard at hand, you can show then the paragraph in a Working Draft. Here is one for c++98, which already had this defined.
You can learn more
here
.