I guess I'll just have to call Frontier and see what their definition of "activity" is. I can probably squeak in a supershuttle trip in April. The March statement email says the miles expire March 31, but the email I got today says "Post activity by June 30, 2012". Confusing.
I spent some time digging around on the Frontier website, and finally found the following in the EarlyReturns membership guide:
Mileage in a member's account will not expire, so long as the Member generates accrual activity in the account. Unredeemed mileage accumulated in a Member's account will expire at the end of the 24 months after the last accrual activity date. For example, the last mileage posting, flight, or partner activity as of May 2011 is valid until May 2013, unless the member has accrual activity in the interim.
Apparently my strategy is not valid on F9 as you must accrue miles to keep the account fully active. It seems that they did push out the expiration date based on my usage of miles, but only by a month. The good news for me is that I don't have that many miles, so if I can't find a supershuttle ride, I wont cry over < 5K miles.