Format string for ISO-8601 date and time

 Hi,

Is there a format string for to_char(timestamptz, text) that would
output a timestamp in full ISO-8601 format? That is, something like
1977-04-22T01:00:00-05:00

I can't find a way to extract the offset against GMT from the docs
here:
http://www.postgresql.org/docs/8.3/static/functions-formatting.html

If not, what would be the way to convert a timestamp to such a string
regardless of the session's datestyle settings?

Thanks,

--
Daniel