I'm less worried about items like this than I am about IR illuminators such as:
Or for that matter a simple IR LED attached to an appropriate power source, and focused slightly. Simple, small, and nearly undetectable unless some one was looking for it.
While interference of other IR signals are an issue, there is real danger in the "Wash-out" effect of a constant on IR source. It would effectively override any and all IR signals within the viewing angle of the IR receiver.
Having used IR in many personal and professional projects in the past, this was one of the first issues that occurred to me.
-S
After re-reading some of the posts here there may be some confusion on how IR and irDA acctually work (ie, refering to frequency etc). Some helpful info can be found in that wonderful repository, WikiPedia at this
Link