Sort of. She writes on her Facebook page:

No longer a Time staffer, but a contractor. More freedom, fewer guarantees, no corp Amex. Net gain, y? Ack. Will cover campaign for food!

Now accepting freelance gigs, paid speaking engagments and babysitting jobs. Also yard work. And may I squeegeee your windshield, sir?

She will also continue to work on her second book.

UPDATE: Just to be clear, she's not technically leaving. We get the official word from Time:

Ana Marie Cox will continue to be a contributor to TIME. It's a change in status, not a change in affiliation, which gives her more freedom to write in other print outlets.