Issue with new site "Oops! Google Chrome could not find…"

I have a new wpmu install (care of wp.mu), upgraded to wp3.0.

The main site is working fine – front end and back. Have created a new test site in the backend: Super Admin > Sites as a subdomain e.g. testsite.mainsite.com

Can see the test site in the Sites list but when I click on the “Backend” or “Visit” I get an error

“Oops! Google Chrome could not find testsite.mainsite.com”

Have tried creating a site again. Also tried with firefox but without luck. Am out of ideas :wink: Anyone got any pointers? I figure I’m missing something obvious…

Thx – Bruce