NSLog’s %s format specifier is in the system encoding, which seems to always be MacRoman and not unicode, so it can only display characters in MacRoman encoding. Your best option with NSLog is just to use the native object format specifier %@ and pass the NSString directly instead of converting it to a C String. If you only have a C string and you want to use NSLog to display a message instead of printf or asl, you will have to do something like Don suggests in order to convert the string to an NSString object first.
So, all of these should display the expected string:
NSString *str = @"你好";
const char *cstr = [str UTF8String];
NSLog(@"%@", str);
printf("%s\n", cstr);
NSLog(@"%@", [NSString stringWithUTF8String:cstr]);
If you do decide to use asl, note that while it accepts strings in UTF8 format and passes the correct encoding to the syslog daemon (so it will show up properly in the console), it encodes the string for visual encoding when displaying to the terminal or logging to a file handle, so non-ASCII values will be displayed as escaped character sequences.