I dont think I have ever flown on Virgin America and probably won't but I'll take the free upgrade to Silver status just in case. View from the wing's blog discusses this further if you want to read more.
The silver status is explained in detail on their website (click here). You can expect to get:
1) 50% more miles
2) Premium check in
3) Better standby chances to get a seat
Well, these are not very impressive but its a free upgrade!