ath5k is part of the kernel since 2.6.25 nothing needed.
Now.. some bugs have been corrected in 2.6.28 so depending on your chipset, it might not work with the stock kernel shipped with Lenny. Even Sid still does not have a 2.6.28 at this time.
In the case of problems,
- either you build your own kernel,
or maybe install one of these (add one of these lines in sources.list)
-
http://wiki.debian.org/DebianKernel
Be sure to have an ethernet connection or an alternate working setup if it fails ;-)
Here is works™