Spirit hasn't really announced this as far as I know, but you can now check in online (their website is not mobile friendly, so its difficult but not impossible to do on the phone) and email yourself the boarding pass.

What I found is that the emailed boarding pass never worked for me (the bar code doesn't display) but if you click the link at the top that says 'Can't View?' it will display a mobile boarding pass.. which also only works about half of the time

But when it does work I save a screenshot to phone gallery, and that gives me something that both TSA and Spirit can scan at the gate. Important since Spirit does charge to print a boarding pass with an agent (still free at kiosk, though there are rumors that may eventually have a $1-$2 charge).

More Info Here: https://customersupport.spirit.com/h...-Boarding-Pass

I remember reading a press release that they would have an app by the end of summer.. no dice there, but at least something...