In the summer of 2008 my wife and I visited 30 zoos and aquariums in an 8 week journey across Canada and the United States. It is well documented on this thread, and there were many avid fans who followed our epic journey. Now there are tentative plans for us (along with our very young daughter - she'll be 10 months old when we plan to leave on July 12th) to take a 5-week trip through the United States to visit loads more zoos!
The trip is outlined below, and probably 90% of it will be accurate. Two years ago there were at least a couple of zoos that we decided not to visit due to timing issues or the weather, and instead we added a number of different establishments, and I can see this potentially happening again. In the summer of 2010 we plan to visit only zoos that we have NOT already seen, and if we do go on this epic trip then we'll have seen over 50 of the top 60 zoos extensively reviewed in the incredibly well-detailed classic "America's Best Zoos", and I'll pass the 100 mark overall for zoos/aquariums/wildlife parks.
I scheduled for 29 days, which leaves 6 additional days for seeing other attractions such as national parks and monuments, as well as time off in case we stretch out the zoo visits with a few days off in between. This is a tentative schedule, but we did an extremely similar trip for 8 weeks in 2008 and so 5 weeks and with a youngster should hopefully be successful.

Without further ado...
Proposed Summer 2010 Road Trip:
Day 1: Drive all day
Day 2: Zoo Montana + drive all day
Day 3: Drive all day
Day 4: Cheyenne Mountain Zoo + drive 7.5 hours
Day 5: Sedgwick County Zoo + drive 3 hours
Day 6: Tulsa Zoo + drive 4 hours
Day 7: Kansas City Zoo + drive 4 hours
Day 8: Saint Louis Zoo + Gateway Arch + drive 4.5 hours
Day 9: Nashville Zoo + drive 3 hours
Day 10: Louisville Zoo + drive 3.5 hours
Day 11: Fort Wayne Children’s Zoo + drive 1.5 hours
Day 12: Binder Park Zoo + drive 4 hours
Day 13: Cleveland Zoo + drive 3 hours
Day 14: Drive all day (5 hours)
Day 15: North Carolina Zoo + drive 3 hours
Day 16: Riverbanks Zoo + drive 6 hours
Day 17: Montgomery Zoo + drive 5 hours
Day 18: Audubon Zoo + drive 7 hours
Day 19: Caldwell Zoo + drive 2 hours + Dallas World Aquarium
Day 20: Dallas Zoo + drive 4 hours
Day 21: Houston Zoo + drive 3 hours
Day 22: San Antonio Zoo + drive 4.5 hours
Day 23: Gladys Porter Zoo + 6 hours
Day 24: Drive all day (9 hours)
Day 25: Rio Grande Zoo + Albuquerque Aquarium + drive 3 hours
Day 26: Drive all day (7 hours)
Day 27: Utah’s Hogle Zoo + drive 5 hours
Day 28: Zoo Boise + drive 3 hours
Day 29: Drive all day (7 hours)
Days = 35 (5 weeks)
Zoos = 23
Aquariums = 2
U.S. States = 26