Yeah the PM G5s (watercooled ones), have a 100% failure rate after 2 years (IIRC), not a good statistic, for a professional machine too...
Although I do think we'll see watercooling in laptops, given apples recent patent we can't be more than a few years off before the tech will need to be implemented, I'm guessing in the 17" MBP/iMac first, then spreading to other/all units over a few years, it's the only way we can keep on pushing CPU speeds up.
EDIT: As A side note, I've been thinking about how difficult it would be to fit a watercooler into a current MBP, I guess once I get a new machine, several years from now, it would be a fun project to attempt, or buy a throw away iBook, if it works, it would be amazing, if not, no big loss. (I'm actually quite competent at CAD programs, and wouldn't be doing this on a "let's get the parts and see if they fit basis", it would be very well organised. Might be fun to try one day.