Following a complaint that the PIV had terrible syscall times in Linux compared to a much slower PIII (this is more an architecture property rather than software problem), Linus implemented a "syscall vsyscall"! vsyscall is basically a kernel supplied page in user address space which a program can call into in order to perform some function. This particular vsyscall determines the fastest syscall mechanism (on P6+ this is SYSENTER, i386 a traditional INT 80, AMD has SYSCALL which could possibly be implemented) and runs it to enter kernel mode. Linus found that this triples the speed of the "NULL" syscall on a PIV. There is a great deal of interesting (100+ posts) implementation discussion following the linked post (somehow eventually degenerating into a BK flamewar again!).
Some people have been concerned about the feature freeze, however it could be argued that this isn't a new feature. It isn't core code with no complicated interdependancies. And the old syscall method is exactly the same, which is possibly slightly slower than Windows XP using SYSENTER in some tests.