The best I can find is this support ticket[0] (Google Cache[1] since it's current redirecting me to a login page) linked from the initial pre-order announcement[2]. The countries are:
* Austria (available as of February 2016)
* Australia
* Belgium
* Canada
* Denmark
* Finland
* France
* Germany
* Iceland
* Ireland
* Italy
* Japan
* Netherlands
* New Zealand
* Norway
* Poland
* Spain
* Sweden
* Switzerland (available as of February 2016)
* Taiwan
* United Kingdom
* United States
* Austria (available as of February 2016) * Australia * Belgium * Canada * Denmark * Finland * France * Germany * Iceland * Ireland * Italy * Japan * Netherlands * New Zealand * Norway * Poland * Spain * Sweden * Switzerland (available as of February 2016) * Taiwan * United Kingdom * United States
[0]: https://support.oculus.com/hc/en-us/articles/216091397-Initi...
[1]: https://webcache.googleusercontent.com/search?q=cache:Igifjz...
[2]: https://www.oculus.com/en-us/blog/oculus-rift-pre-orders-now...