is NSDateFormatter dateFromString: supposed to return nil for an invalid string?

Question

According to the class reference, dateFromString method returns "A date representation of string interpreted using the receiver’s current settings". Using the following code:

NSDateFormatter * formatter = [[[NSDateFormatter alloc] init] autorelease];
[formatter setLocale:[[[NSLocale alloc] initWithLocaleIdentifier:@"en_US"] autorelease]];
[formatter setDateFormat:@"MMddyy"];
NSDate * date = [formatter dateFromString:@"2010-04-27T17:45:44.763"];

in SDK 3.1.3 returns nil, but on SDK 3.2 it returns 2010-01-20 00:00:00 -0300. Is the 3.2 version the correct behavior?