First install the firmware:

 apt-get install firmware-iwlwifi

Next, rebuild the kernel and reboot (this is not strictly necessary, but it is what I did; other people can tell you other ways), so that you have the source for the kernel you are currently running available.

Download the latest kernel module source from intellwireless.org

Unpack, make (probably twice) su, make install

Update

To avoid recompiling your kernel, you can follow the steps from Stuart Prescott