From the website:

"Starting with your very first flight, you'll earn 5 points for every $1 you spend for travel on Virgin America. Then, in the coming months, you’ll be able to redeem a free trip for as few as 4,900 points. Earn a free flight after as few as 4 roundtrips!"

My concern is with "as few as 4,900 points"? Is that maybe for a short-haul (e.g. SFO-LAX) flight, or could it also be for a SFO-JFK trip? If so, I would consider purchasing the discounted F fare for a SFO-JFK r/t since that would immediately earn enough points for another SFO-JFK r/t. 2 SFO-JFK roundtrips for $890, with one in F...not too shabby.

But I'll await some clarification from VA on this...