The official (I assume given it's a branch of the mono git hub repository) monotouch-bindings project over on github now has support for the TestFlight SDK, check out:
https://github.com/mono/monotouch-bindings/tree/master/TestFlight
and
https://github.com/mono/monotouch-bindings
Excerpt from their readme:
TestFlight
This is a MonoTouch binding for the TestFlight SDK which can be found at
https://testflightapp.com/sdk/
The current version of this binding is for TestFlight SDK 1.2.1
v 1.2.1 released on 26 dec 2012
Bindings updated on 17 mrt 2013
Building
Run make
in the binding directory to build TestFlight.dll
Using TestFlight.dll with your own iOS App
Simply add TestFlight.dll to your project's References in MonoDevelop and you
are good to go!
To use the thread safe TakeOff methode. Change TestFlight.TakeOff(token) to TestFlight.TakeOffThreadSafe(token)