Out at lunch again today and found the same thing, as you say in the 35-40mph range. It really gets mediocre FE at this speed with the ICE running so I said screw it and jumped it up to 50/55mph (fortunately its one of those uncongested 40mph roads where most people drive it limit + 10 often). Got really good FE at this speed. I would hazard a guess that the A/C does not have a big impact in situations where the ICE would normally run anyway, but is a real killer at 35-40mph as it really does make getting a good E-mode run a pain in the rear. Doing the 25mph and below E-mode where appropriate seems to work the same with or without A/C. I think I've added a new strategy to my hybrid driving style, when in A/C if the option is available 50mph is going to do better FE for me than 40 does. Opposite when A/C is off.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters