Most of the Linux input subsystem development is being conducted in the main 2.5 BitKeeper kernel. BitKeeper is a very powerful system, although if you simply want to browse, linus.bkbits.net:8080/linux-2.5 is a useful starting point.
There is a “bleeding edge” development tree just for the input subsystem, available at linux-input.bkbits.net:8080/linux-input.
linuxconsole.sourceforge.net was the main development tree for the Linux input subsystem, but the kernel development has transitioned to BitKeeper. While the site includes little documentation on the input subsystem, there are some useful patches for userspace applications in the CVS tree.