I had read about this as well, and recently had the auto transmission and differential fluid changed. I can't believe any oil/fluid will last forever. I took my C240 to a local Mercedes specialist and it cost about £100 to get them both changed - a lot cheaper than a new tranmission!
I've just had both changed as well - car was at 85,000 miles and I'd like it to go a lot further WITHOUT my having to buy a new transmission or have hte back axle rebuilt.
"Sealed for life" transmissions and axles are exactly what they say - you don't have to bother doing anything until the transmission/back axle packs up - a lot earlier than it would do otherwise.
I'm going to follow my local Mercedes independent's advice, which is to change the oil/fluid every 40-50K
