The 2.6 Linux kernel will support 2 fully POSIX-compliant implementations of threading. The one which is likely to be the default maintains the one thread, one kernel thread situation. However the simplicity of this arrangement also helps make it scale very well. See Native POSIX Thread Library 0.2 Released and Native POSIX Thread Library 0.1 Achieves 100,000 Concurrent Threads for more detailed information.