XLNT observation & question, Goeytex. Per nick, typical receivers lose sensitivity off their center frequency. TV remotes blast a ridiculously strong signal, repeating after a pause as long as you hold the button. That intensity bounces off walls & ceilings and a saturates the IR modules to make bulletproof communications. Even 56 kHz IR receiver modules get swamped by a 38-40 kHz TV remote.
If you use a safe value of series resistor (max current=20 mA) to drive your IR LED directly from an IO pin, you'll have a relatively weak signal that might be affected by the wrong mod frequency. Options for a stronger signal include reducing the series resistor (OK for regular duty cycle, but risky for the IO pin if it gets locked on accidentally) or using a transistor switch to drive the IR LED.
Of course another factor is the wavelength of the particular IR LED you use, which should match the IR receiver. And there are dozens of different types of IR receivers, each has its own characteristics, some filter sunlight and noise, some are continuous signal compatible. They all look nearly the same but behave differently. Radio Shack sells two completely different modules (physically and electrically) in the same package, 276-640. The metal frame units are continuous signal compatible, the other is not.
I'll be doing some experiments with IROUT shortly, I'lll be interested to hear your findings.