to!string() seems to produce wrong results with -O using current DMD here on my OS X Lion box:

---
import std.conv;
import std.stdio;

void main() {
  writefln("%s %s", long.min, to!string(long.min));
}
---

If I build the snippet without compiler flags, it prints (as expected):
-9223372036854775808 -9223372036854775808

If I add -O, however, the output becomes:
-9223372036854775808 -9223372032559808512

It seems like the lower 32 bits are zeroed out in the to!string() version (the problem occurs also when to!string is used in isolation, the writefln is just for demonstrative purposes).

Interestingly, this only occurs on my OS X box, I can't reproduce this in a Linux VM (neither on x86 nor x86_64).