Given a DateTime object, how do I get an ISO 8601 date in string format?

Given:

DateTime.UtcNow

How do I get a string which represents the same value in an ISO 8601-compliant format?

Note that ISO 8601 defines a number of similar formats. The specific format I am looking for is:

yyyy-MM-ddTHH:mm:ssZ

Note to readers: Several commenters have pointed out some problems in this answer (related particularly to the first suggestion). Refer to the comments section for more information.

DateTime.UtcNow.ToString("yyyy-MM-ddTHH\\:mm\\:ss.fffffffzzz");

Using custom date-time formatting, this gives you a date similar to
2008-09-22T13:57:31.2311892-04:00.

Another way is:

DateTime.UtcNow.ToString("o");

which uses the standard “round-trip” style (ISO 8601) to give you
2008-09-22T14:01:54.9571247Z.

To get the specified format, you can use:

DateTime.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ")