Thomas describes this proposed performance counter subsystem as being very simple (it only takes a few lines of user-space code to read the counters) but still an extensible design that can implement a full range of features. Also posted on the Linux Kernel Mailing List was a simple monitoring demo. Thomas believes that the design of this subsystem is superior to that of some of the other recent patch sets that add similar functionality. However, there still is quite a bit of work left to be accomplished for this performance counter subsystem. Right now Intel Core 2 and newer CPUs are supported with their performance counting registers, but beyond that there isn't any non-Intel CPU support.
Of course, being the Linux benchmarking fanatics that we are, we'd love for this to enter the mainline Linux kernel soon. It would also be fun to add a module to read these counters for the Phoronix Test Suite. To find out more about the performance counter subsystem, checkout this LKML thread.