Wow, clever use of _msize(), I hadn't thought of that.  This is a much 
cleaner approach then the original and likely to be more robust.
There is one problem though:
Under VS2008 (and probably 2005), _get_osfhandle(fd) on an invalid fd 
will cause the assert that we are trying to avoid!  Please do this only 
if you deem the fd to be "valid".
If you fix this, it has my vote.