Thanks to all of you for your very thoughtful responses. The writings of John Deakin that Mike mentioned are absolutely phenomenal. Finally, I feel like I understand a little about the subject of engine management. Interestingly, it is his opinion that the problem with Continental engines eating up their jugs is related more to manufacturing and mechanical issues than it is to pilot technique, as Chief alluded to.
Yes, it is the valves that are leaking. The 150 hours is only since the last inspection, and as I just discovered, the Millennium jugs themselves have around 900 hours on them since new, when the engine was major overhauled in 1997.
According to Deakin, the issue is most likely with poor lapping of the valves during manufacture or overhaul, or poor valve guide/valve stem alignment. This results in areas of the valve that do not properly contact the seat to enable proper cooling, and eventually the valve warps, or worse, breaks and the top gets sucked into the cylinder. We all know what comes next after that happens...the pilot gets to build some glider time. Surprisingly, CHT, EGT, and leaning really don't have all that much to do with it, unless at the extreme, of course. That's not to say that proper engine management isn't critical, just that other factors have a greater impact on the life of the engine.
Chief, I really hadn't given much thought to the mags, but the mechanic mentioned that one mag has a pretty good drop- more than it should. He also said that the plugs are at their life expectancy limit. Dunno what type they are. Have you experienced significant problems with engine durability when the plugs were worn?
What's so hard for me to fundamentally understand, is why aircraft engines are so much more sensitive than other air-cooled piston engines. How can a VW bug or a Porsche Boxster run every day, accumulating perhaps 200,000 miles, running for 6000 or 7000 hours at 3-5000 RPM, with absolutely no attention given to mixture or CHT, without ever encountering valve problems? But a $30,000 Continental can't run 650 hours? Is the quality of workmanship and materials in American made aircraft engines really that bad?
Anyway, for those who are interested, here are 3 links to Deakin's articles that are WELL worth a read, if you fly or maintain piston-driven aircraft. Great stuff, and thanks again all.
Manifold Pressure Sucks:
Putting it all Together: