“New real-time Linux enhancements open a whole new world of possibilities for Linux, ranging across the latest 3G technologies and as near as the mobile handset in your pocket. The purpose of modifying the Linux kernel with real-time functionality: to dramatically reduce interrupt and task preemption latency, thus enabling the 2.6 kernel for use in high-performance multimedia applications and those requiring extremely fast, task level reliable control functions. Real-time Linux has come a long way – where is it now and where is it heading?”
It is about predictability (i.e. what is the worst case execution time of a computation). You can build a hard real-time system with very long (but known and bounded) interrupt latencies.
The Linux real-time patches try to make it predictable (hence, the O(1) computations, etc.)