Sorry for bringing up an old thread, but I'd just like to add my personal experience with this. Long story short, I recently took up the practice of engine braking on an automatic minivan, and my transmission is now dead at 100k miles. Could just be a coincidence, but here's the long story for what it's worth:
On a drive up and down the Rockies a bit, 1 year ago, I decided to try engine braking while heading down hills. We're talking about 40-50 mph in 4th gear (the gear just below my OD) and 30-40 mph in 3rd. 3k rpms max, usually closer to 2k. After about 1.5 hrs of doing this on the way back, I noticed a whirring sound. It worried me, but after getting home and checking the transmission fluid, everything seemed fine. I figured I was just being paranoid, that the sound had already been there and that I was just worried that after trying this new thing I was prone to think I did something wrong. But...
After about a year of frequently doing this, not just in the mountains, but at red lights, etc., the 3rd gear suddenly now has a terrible time shifting. And 4th seems like it's about to go as well. The transmission mechanic says the fluid looked fine and everything else is checking out. My "clutches" are going out, he says. Time for a transmission rebuild.
I'm sure it's not that rare for a 10 year old transmission with 100k miles to go out all of a sudden, but I can't help but think that it was the engine braking that made it happen so soon. Whether or not the whirring sound was already there will remain a mystery, but I'm fairly certain it wasn't. I don't want to build unfounded fear for engine braking, but the saying "better to wear out a $150 part than a $3000 part" is really hitting close for me.