There are some general RealTime operating systems that support PIC, and a few PIC-specific RTOS's out there. For now, our applications are simple enough, and we need to be so close to the hardware, that we'd rather have the speed and transparency of a framework (academics call this an "exokernel") than the complexity of an OS. But these do look interesting!
- Salvo RTOS: http://www.pumpkininc.com/
- uC/OS-II claims to have a PIC port, but I'm pretty dubious: http://www.micrium.com/
- PICOS18: http://perso.wanadoo.fr/softelec/Projects/RTOS/PICOS18/Projects_RTOS_PICOS18_us.htm
- Some relevant Microchip application notes: