Replace intel below with your favorite driver name.
To build an input driver such as xserver-xorg-input-mouse, replace video-intel with input-mouse below, and install in /usr/lib/xorg/modules/input/ instead of /usr/lib/xorg/modules/drivers/.
Testing a new upstream git snapshot of the X.org driver may help. It happens for some other drivers too. Instead of waiting for us to package a new version, people can easily test development drivers without having to build a new package or so. Since there is a single (or very few files) file to install, you can just build the upstream source and install manually. Here's what to do for the Intel driver.
You'll need the git source control management tools, some Xorg related macros and some development headers/libraries:
$ apt-get install git-core xutils-dev autoconf automake libtool
$ apt-get build-dep xserver-xorg-video-intel
Then you can grab the git tree and enter it:
$ git clone git://anongit.freedesktop.org/git/xorg/driver/xf86-video-intel
$ cd xf86-video-intel
If you need to try a special branch (for instance xf86-video-intel-2.2-branch) instead of the master branch:
$ git checkout -b xf86-video-intel-2.2-branch origin/xf86-video-intel-2.2-branch
If you have to test a patch mypatch.diff:
$ patch -p1 < mypatch.diff
Then you build with:
$ ./autogen.sh
$ make
Once it is built, run make install prefix=/usr as root (or copy src/.libs/*_drv.so in /usr/lib/xorg/modules/drivers/ (or /usr/lib/xorg/modules/input/ for input drivers)) and restart X (logout/login is usually not enough, Ctrl-Alt-Backspace or xdm/gdm restart is ok).
To revert to your old driver, just reinstall its package with aptitude reinstall xserver-xorg-video-intel or dpkg -i xserver-xorg-video-intel_foo.deb.