There exist a few patches that may improve performance of the host, or add useful features that will help with emulation.

High resolution timers will help your guest system's clock accuracy and improve overall performance by reducing system load:

http://www.tglx.de/projects/hrtimers/2.6.22/patch-2.6.22-hrt6.patch

Patches for other kernel versions can be found in a parent directory:

http://www.tglx.de/projects/hrtimers/

If you plan to use a partition-backed disk image, you may be interested in improving the security of your data. Download loop-aes from sourceforge and apply the kernel patch from:

http://loop-aes.sourceforge.net/loop-AES-latest.tar.bz2

In addition to the following patch, you may want to modify a udev rule in:

/etc/udev/rules.d/50-udev.rules

find the line that contains "tun" and make it look something like this:

KERNEL=="tun",          NAME="net/%k",  GROUP="qemu"    MODE="0660"

There also exists a patch that enables group ownership of tunnel devices, which may be beneficial in a multiuser environment (where the users trust each other). Note that the kernel patch will be in in mainline as of 2.6.23. Download the kernel patch here or here and the userspace patch here or here.

The kernel patch applies cleanly against linux 2.6.22 and the userspace tunctl patch applies cleanly against uml_utilities_20060323.