Go here:
http://www.linux-sxs.org/administration/25xconf.html
~snip~
3. What you need to use the new kernel
module-init-tools:
http://www.kernel.org/pub/linux/kern.../rusty/modules
Kernels 2.5.48 onward have depended upon module-init-tools due to massive changes to how modules are loaded and unloaded. During installation of module-init-tools (unless emerging it on Gentoo when this is done for you automagically), type 'make moveold' to rename the old insmod, modprobe, rmmod, and lsmod to insmod.old, modprobe.old, rmmod.old, and lsmod.old, and does the same with matching manpages. 2.4.x kernels should then automatically use the old programs.
Other than that, a willingness to live with bugs is helpful.
I encourage anyone who's ever thought about 'living dangerously' with the very core of your OS to try it now, and those who haven't thought about it to do the same. Overall performance has been zippier for me, and the annoyances of separately patching and installing for XFS and alsa are taken care of.
If the standard 2.5.x tree doesn't work out for you (esp. if you have a VIA chipset with USB as I do) I recommend Alan Cox's -ac tree:
http://www.kernel.org/pub/linux/kernel/people/alan.