I've got the AD7789 ADC wired up to my board, and the crazy thing about this part (and some other ADCs) is that there is no separate DRDY pin, it's dual-functioned with MISO. I get "An unhandled exception of type 'System.InvalidOperationException' occurred in Microsoft.SPOT.Hardware.dll", so obviously the compiler is complaining that I'm trying to declare the MISO pin as an InputPort as well.
I tried wiring MISO to a GPIO pin so I could monitor that, but it didn't work. What about Tristateport? I don't have much experience with them, so I don't know if that's the answer.