This should do the trick:

NSString * ReplaceFirstNewLine(NSString * original)
{
    NSMutableString * newString = [NSMutableString stringWithString:original];

    NSRange foundRange = [original rangeOfString:@"\n"];
    if (foundRange.location != NSNotFound)
    {
        [newString replaceCharactersInRange:foundRange
                                 withString:@""];
    }

    return [[newString retain] autorelease];
}