It does seem that way, PremEx. I cleared four days out on SEA-IAD and I was the only person in the cabin at the time. I grabbed the only available upgrade (of 24 available seats) on a 757 IAD-SEA three weeks out. My IAD-LHR cleared at booking, but LHR-LAX cleared at that gate (and I was the last one to clear) with 14 open three days out.

I like UA is making Z fares right there with B fares, as it is getting me to buy the Z for a few hundred more and save the miles. Plus I get the pick of the cabin a few weeks out.