Yes, it's the equivalent to BA's I class fare. It can be booked through travelocity, expedia, all the normal onliners and of course Virgin themselves. (By phone, however, - ASK for a Z-class fare, rather than the "cheapest" which is usually issued in D)
I think it's a 42 day advance purchase. The ticket is completely and utterly non-changeable and you're unable to cancel it without losing all of your money.
If, however, your travel plans are fixed, it's a very good deal - Upper Class to San Francisco for less than £2k...