Unless something has changed from last year, you don't actually have to file by the deadline, you just have to pay your TurboTax fees by then, but can file later. Last year, the deadline was later (March 15 or 16, I think). Once you pay, they will not require more, unless you add services (i.e., didn't initially pay for state filing and you use that). There's a bit of a trick to paying without filing...Open your current return, bring down the tax tools menu, and click on print center, then on print, save or preview this year's return, then on the pay now box. You need to pay before it will let you preview/print the return, but won't make you file...you can just do that when you're ready later.
I almost always need to do this, as I have foreign income I have to report but I don't usually know the specifics until late March - this is the way I've used to still get the discount rates while filing later.
