I agree that the back pages of Flyer Talk are all you really need by way of research.
Surprisingly, I was able to upgrade with no problem on four Delta flights last week - DCA/ATL/IAH and IAH/ATL/DCA. On the first leg, I simply forgot to call until a couple of hours before flight time (I'm a lowly Silver by virtue of my Million Miler status and have to wait until 24 hours out to use segments). No sweat on both legs. On the return, I remembered to call. Again, no sweat. Both flights were full in both cabins. Has Delta listened to us and made more seats up front available for upgrade, or was I just extremely lucky?