First I thought it was a simulator bug and uploaded to the device. But when I reproduced the same behavior there as well I started to think how to get rid of such behavior (since it was essential for me). Came up to such a solution:
[self.navigationItem setLeftBarButtonItem:[[[UIBarButtonItem alloc] initWithCustomView:[[UIView new] autorelease]] autorelease]];
And to show the back button again you write:
[self.navigationItem setLeftBarButtonItem:nil];
That's simple. Use it as a work-around, guys! Very strange this bug survived even in iPhone OS 3.0...