Alas, that offer has expired, but you can still join the Inner Circle for cheap: It's now $1 per month for the first six months.
However, there's the same small catch: If you're bringing your own iPhone, you also need to bring a phone number from your previous carrier. That's fine if you still have active service, but what if this is a phone that's been sitting in a drawer for six months? What if it's one you bought used to use as a backup? Now you've got no number to port.
Indeed, without a number, you can't even complete Virgin's eligibility check, meaning you can't sign up for service. So you're out of luck, right?
Not necessarily. I was able to work around that limitation by porting my Google Voice number. Here's how you may be able to do likewise:
Step 1: If you don't already have one, get a Google Voice number. It's free.
Step 2: Unlock the number so it can be ported to Virgin Mobile. You can do this by visiting www.google.com/voice/unlock and following the steps there. There's a $3 unlocking fee. Once you receive the confirmation message, take a screenshot. Virgin didn't require me to show any proof of ownership of the number or proof that it was unlocked, but it's a good thing to have just in case.
Step 3: Call Virgin Mobile (888-322-1122) and ask to sign up for an Inner Circle plan. (Alas, you still can't sign up online, as the Google Voice number isn't technically registered to your phone's IMEI number.) When the representative asks for your phone number and carrier, say it's Google Voice.
And that's it! I actually had to go through this process twice, for reasons I won't bore you with, and both times the rep had no problem processing a Google Voice number. Your mileage absolutely may vary, but I'm living proof that it's possible to port a Google Voice number to Virgin Mobile and get an Inner Circle plan as well.
If you do end up having a different experience, hit the comments and explain what happened.