RunDouble has this feature, though they label it as experimental.  I haven't used it myself, so I can't give you any first hand experience with it...

I'm doing speed work, and I'd like to set up my phone to beep at me if I drop below 8 mph. 

I currently use sportstracker, but I'm not seeing anything that would work that way. Closest is the feedback at every lap, but that's not really what I'm looking for.

Does anyone know if I'm just missing it, or if there's another app that would do this?

Question about running apps