would trying to detect a 940nm IR LED with a 930nm phototransistor be possible?
Thanks for the circuit oddbot, i'm guessing that you power the IR LED just like a normal LED?
Yes, although you may be able to feed more current (smaller resistor) to the IR LED compared to a normal LED. 20mA is an extremely common LED forward current rating, but many IR LEDs can take 40mA, 50mA, or even higher currents which will increase the power output.
Ok. what type of circuit do i have to make to be able to detect it with a picaxe?
This is the circuit I use for all my infrared sensors now. You may need to experiment with the value of the 10K resistor depending on the phototransistor your using and the brightness of the IR LED.
Vambo is right. From my experience it would work fine.
That's entirely possible, any drop in efficiency should be negligable. Get the data sheet for the photo trans, it should contain a spectral response curve. This will show the drop in efficiency vs wavelength. I doubt you'll see much (if any) difference between 930 and 940nm.