Rear seal and valve cover leaks at 40,000 miles?

What is your question?

I've had a CLK 430 and now own a SL 500. For both cars at the 40,000 mile service B the dealers (2 different ones) reported valve cover and rear seal leaks. The CLK 430 was under warranty and they claim to have replaced it. The SL 500 is not under warranty. When queried, the service advisor stated it was just a few drops but would get worse over time. How common is this problem?