thanks for all the responses. looks like it varies a lot, but failure at 40k seems conceivable.
i asked my SA if there were ever any warning signs or maintenance methods for preventing or replacing the water pump before it failed, but his response was "it's electric - it will fail without notice and that's why you have AAA." there were no warning signs for me until the high temperature warning started coming on.
guess that is indeed nature of the beast with this particular car and its cooling techniques. a quick search seems to indicate that that bmw water pumps tend to be problematic in general. it's definitely a pain, but the looks on the faces all the SAs when a virtually intact, newish car is being towed in... kind of priceless.
(i am kind of the anti-poster child when it comes to positive advertising for BMW... back emblem got ripped off in oakland, and it's already been towed in numerous times for HPFP problems. damned pumps)