Interestingly enough malloc(3) on Lucid Lynx box says:

       malloc() allocates size bytes and returns a pointer  to  the  allocated
       memory.   The  memory  is  not  cleared.

whereas Lion says nothing particular about it

So I wrote a quickie:

    #include <stdlib.h>
    #include <stdio.h>

    int main() {
        int m = 1024*1024; //1M
        int t = 1024*m; //total count: here 1G
        char *v = calloc(1, t*sizeof(char)); //total size: 1GB
        for(int i=0; i<t; i++)
        {
            if (v[i] != 0)
            {
                printf("%016u @%x %x\n", i, v+i, v[i]);
            }
            else if (i % (t/10) == 0)
            {
                printf("%016u @%x\n", i ,v+i);
            }
        }

    }

Ran multiple times with various sizes including big enough sizes (t) to statistically cover a good deal of physical RAM, and the trigger never triggers.

I notice though that on Mac OS X the program starts right on the spot, while there is always a constant delay on Lucid. Looks like one zeroes on demand (by chunks probably) whereas the other zeroes initially.