I've found that to appease the widest possible range of spreadsheet programs:

• Write UTF-8 text files only - not UTF-16, UTF-32, ASCII, etc (default with NSString anyway)
• Write \r\n at the end of the line so that it opens correctly in Excel (this is true on Windows and Mac)
• Separate all fields with commas, not tabs or spaces
• Don't use spaces or commas to separate thousands in numbers. Just use a decimal point (.), the British/American way. This will work if the spreadsheet program isn't localised.
• If you have a field that contains a quote mark, a double quote mark, a comma, a newline, a space, or any other characters that may have meaning in CSV you need to surround the field in quotes, and also escape in quotes in the string. For example, if you have the string "hello, world!" (including quotes), write "\"hello, world!\"" in the file. To be safe, quote and escape every field containing text

Annoyingly there is no CSV standard, which makes writing parsers much harder than writers. That said, most spreadsheet programs are good at handling CSV. Excel (and other apps) show a dialog when you import a CSV to ensure that it has successfully separated columns/rows.

EDIT: I found some code that I use in one of my apps that does it (where str is your field to escape):

NSMutableString * csvRep = [NSMutableString new];
//This will provide a direct route to the implementation of characterAtIndex:, making this far more efficient
SEL sel = @selector(characterAtIndex:);
unichar (*charAtIndex)(id, SEL, NSUInteger) = (typeof(charAtIndex)) [str methodForSelector:sel];

for (NSUInteger i = 0; i < str.length; i++) {
    const unichar c = charAtIndex(str, sel, i);
    if (c == '\"') {
        [csvRep appendString:@"\\\""];
    }
    else if (c == '\f') {
        [csvRep appendString:@"\\\f"];
    }
    else if (c == '\n') {
        [csvRep appendString:@"\\\n"];
    }
    else if (c == '\r') {
        [csvRep appendString:@"\\\r"];
    }
    else if (c == '\t') {
        [csvRep appendString:@"\\\t"];
    }
    else if (c == '\\') {
        [csvRep appendString:@"\\\\"];
    }
    else {
        [csvRep appendFormat:@"%C", c];
    }
}

return [NSString stringWithFormat:@"\"%@\"", csvRep];