I'm assuming it changes the idt dpl in usermode...
If u have vmware, can u try:
mov ax,ds
test ax,4
jnz ...
And tell me if there's any difference? (in normal NT, bit 100b of ds = 0).
I havent traced much after the ioctl 1800, probably later I might be able to
answer some of those questions
laters

EDIT: according to the thread replies I've received on the other thread,
it seems to be a 9x\NT detection code...
thread link is:
hxxp://www.exetools.com/forum/showthread.php?t=6427