That being said, your statement is unfounded. If you're LIFTING, and lifting HEAVY, then yes, it doesn't make sense to go for more than 45 minutes, for exactly the reasons you stated.
However, cardio? People will see huge cardio benefits if they run for a long time. There's no "negative fitness effect."
Yes, you may burn muscle, but it's not like you're going to all together stop burning fat. In fact, when you hit glycogen depletion (the 'hitting the wall'), your body will switch energy burning modes entirely, and start burning a TON of fat.
I'm nit-picky about your statement because America, as a whole is in such a crisis. We look for any excuse to get out of exercise. Those idiots at the gym lifting 10 lb weights? Do you really think they're getting a good workout? They're not lifting heavy, and they can go far longer than 45 min doing 7 lb arm curls. What about those on the treadmill, walking for 45 minutes while talking on the cell phone?
As a society, we've become too used to everything being easy- we want an easy job with lots of money, we want to consume everything, and we want to be able to eat what we want, and just have the pounds come off.
Losing weight is hard. It takes discipline and focus. Now, especially in the two weeks of the Olympics, we should be able to collectively get off our fat asses and look at what some Americans have been able to accomplish with a ton of hard work and sacrifice.
Think Michael Phelps works out 45 min a day? All those gymnasts? Tyson Gay? So maybe you should step back, realize there's more to life than T-nation, and look at actual exercise science.
T-Nation is the equivalent of looking at some IT trade magazine talking about Java applets and their Web 2.0 capabilities. Yeah, it can be done. Is it the best tool for the job? No.