I don't have a horse in this race, but since I lived in NY for a while. Here's my $0.02. IMO, Grand Central is a more typical mid-town reference than Time Warner. I always put Columbus circle on the cusp of upper-west and mid-town. When you hit the bottom of Central park, your getting into the upper sides. In fact, Columbus circle is just a brief walk to where Amsterdam used to beav84fun said:OMG!!!
Another poster used the Time Warner Bldg as a mid-town reference so lets go with that.

When I was there, we used to refer to downtown as lower manhattan. If you went below midtown, you went by area: Chelsea/Grammercy (which is actually in midtown), then the Village, then lower east/west, and then downtown/financial district. If you went downtown, you were WAY below 14th. For the visiting folks, I'm sure dividing Manhattan into 3 areas (upper/mid/down) is probably good enough. But if you live there, you need a little more info to get around.
The cool part is that my wife used to live in Murrah Hill, which was nice when I went to Corner. Oh the good ol' days...
-td