Check out MontaVista's Linux Real-Time characterisation projects at http://www.mvista.com/realtime
For many applications a soft real time user space thread is adequate, particularly if you minimise kernel scheduling latency with the patches at: http://www.redhat.com/~mingo/lowlatency-patches/
Also, see: http://lists.linuxppc.org/listarcs/linuxppc-embedded/199912/msg00006.html
If your application requires a hard real time response with guaranteed low latency, you may need to encapsulate the real time aspects in a device driver. This is how devices which have real time constraints such as serial ports and disk drives are traditionally handled under most non-realtime operating systems, including Linux.
The real-time response of a driver will still be affected by other kernel code which may disable interrupts for unknown periods, increasing effective interrupt latency. For some tools to help measure interrupt latency, see: http://www.drfruitcake.com/linux/irq_blk.html
If you need better latency guarantees than offerred by the kernel, use Real Time Linux to decouple the hard real time portion from the rest of your application. v3.0 includes support for PowerPC. See: http://www.rtlinux.com/
Also, search for RTLinux.