I think Ubuntu 9.04 may have the ath5k atheros driver which is not as
compatible with older chipsets.

So you may have to blacklist the current driver in Ubuntu 9.04 and build
the older stable version for the madwifi-project.org website:

http://madwifi-project.org/

I had to download and install a replacement ath_hal module to get the
Atheros card working on my wife's HP laptop. She is running Ubuntu 8.04 and
here wireless card does not work with any of the std Ubuntu kernels.