Thanks for the tip WaKaRu, but that doesn't actually affect piKey.
All PRX's are in kernal mode. Moreover, the only differance from porting a 3.XX kernal applicatin to a 4.XX kernal application is the makefilel line:
PSP_FW_VERSION = 401
However, that's only needed for EBOOTs.
Florians problem is probably due to either due to Unofficial piKey 0.4 getting old, or IRDA.PRX is missing from his flash0.
At the weekend, I'll look into it.