Here are my experiences so far this month:

LGA-ATL on 8/5: Successful, even though when I got to the gate they were just starting to announce pre-boarding. (Biz class ended up filled)
ATL-LAS on 8/5: Unsuccessful - biz class was already full

I ended up using 4 credits to upgrade both of my return segments.

Since the "free upgrades at T - 40 minutes" policy started, I have gotten somewhere between 6 and 10 free upgrades, mostly on LGA-MKE & v.v. and HPN-ATL & v.v, with a few outliers. Once, coming back from PNS (or maybe JAX?), I walked up to the gate roughly 45 minutes before departure to ask about a complementary upgrade and the GA had already done it for me!

One last note: When flying two segments on one ticket, I almost always use 4 credits to ensure an upgrade - especially to the west coast. On my half a dozen or so cross-country flights on AirTran, biz class has always been full. I would be very surprised to hear of anyone getting a free upgrade out west to or from ATL, but maybe those of you routing via MKE will have more luck.