string txt= "2009-10-08 08:22:02Z";
DateTime output = DateTime.ParseExact(txt, "u", System.Globalization.CultureInfo.InvariantCulture);
The DateTime class supports the standard format string of u for this format
I think for the ISO format (with the T separator), use "s" instead of "u". Or use:
string txt= "2009-10-08 08:22:02Z";
DateTime output = DateTime.ParseExact(txt, new string[] {"s", "u"}, System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None);
to support both formats.