Postdoc recommendation letters: The European-American gap and how to overcome it
Is this cultural difference taken into account by members of postdoctoral search committees in high-ranked American universities?


Yes, certainly.  If your letter writers are sufficiently well known, then there's no issue, since the letters they have written in the past will serve as calibration.  (If someone shows poor judgment in their letters, then committees will start discounting the letters, but committees can and do adapt to good judgment expressed in unconventional ways.)  If the committee members haven't seen many previous letters from someone, then there's more of an issue, but everyone will be aware of the phenomenon of European-style letters and will try to take that into account.

As a general rule, letters still need to offer concrete details and say something substantive.  A very short letter along the lines of "This young mathematician does solid work and would make a reasonable postdoc at University X" won't help, because it offers nothing but a vague sense of approval.  However, a substantive letter will be useful even if it's more understated than the U.S. norm.


  I'm currently gathering my recommendation letters, and I'm supposed to get one from one of the leading figures in my field. He knows my work well, and I believe he has a good opinion of me. On the other hand, being familiar with his style of expression, he will never write one of those "omg he is the best researcher ever" letters that top American universities expect to see (he won't write it even for someone who is actually the best researcher ever). 


This doesn't sound like a problematic situation to me.  If you're worried, you could look into whether he has ever had a student or postdoc who subsequently got a job in the U.S.  (Presumably the answer is yes, in which case his letter must not have ruined the application.)