Per the nodejitsu FAQs, if you are on a personal plan, your host is Joyent's us-east-1 datacenter in North Virginia (USA). Business plan users are able to choose from additional datacenters which include Joyent's us-sw-1 in Las Vegas (USA), eu-ams-1 in Amsterdam (Netherlands) or Telefónica's eu-london-1 in London, England.
Looks like additional datacenters will include Joyent San Francisco (USA) and Telefónica in Madrid (Spain).
Update:
Nodejitsu recently experienced an extended outage which is documented in detail on their blog. A consequence of this was to drop support for Telefónica Instant Servers.