K&R's itoa implementation example (page 64, in case anyone is curious) specifically calls out that it doesn't handle INT_MIN, and explaining why and fixing it is one of the exercises.

Koenig's C Traps and Pitfalls does the opposite of what you'd expect by making the recursion work with negative integers and reducing the positive case to that. That in turn exposes the issue of C having an implementation-dependent meaning for x / y and x % y when x is negative. But he works around that in the usual way.

This solution doesn't assume 2's complement, but it does assume that the negative range of integers is at least as large as the positive range. This works with 2's complement, 1's complement and sign-magnitude, so it's pretty damn bulletproof. It wouldn't work with a hypothetical representation where the positive range was larger than the negative range. Making this fully portable to any ANSI C conforming system is possible (work with either positive or negative integers depending on which has the larger range) but it involves the chicken-and-egg problem of portably determining whether INT_MIN or INT_MAX is numerically largest. Here's one way to do that (untested):

        div_t quotrem(int numer, int denom)
	{
	    div_t d;
	    d.quot = numer / denom;
	    d.rem = numer % denom;
	    if (d.rem < 0) { d.rem += denom; d.quot--; }
	    return d;        
	}

        int abscmp(int x, int y)
	{
	    if (x == 0 && y == 0) return 0;
	    if (x == 0) return y > 0 ? -1 : +1;
	    if (y == 0) return x < 0 ? -1 : +1;
	    div_t dx = quotrem(x, 2), dy = quotrem(y, 2);
	    int r = abscmp(dx.quot, dy.quot);
	    return r == 0 ? dx.rem - dy.rem : r;
	}

Hanson and Fraser's A Retargetable C Compiler does the inner loop the usual way with positive integers but using unsigned arithmetic. They specifically handle the INT_MIN case with code along these lines:

        if (i >= 0) u = (unsigned) i;
	else if (i == INT_MIN) u = 1 + (unsigned) INT_MAX;
	else u = (unsigned) -i;

Here's another solution I just thought of that should be portable to any ANSI C conformant platform. Write the usual divide-by-base-until-zero loop but use this function to determine the right digit to put into the string buffer:

        int digit(int x, int base)
	{
		div_t d = quotrem(x, base);
		return x >= 0 ? d.rem : base - d.rem;
	}