Apple Magic Mouse scrolling in Ubuntu 9.10
With kernel 2.6.34 the Magic Mouse driver is included, so scrolling should just work(tm).
I have Ubuntu 10.04 and updated to linux-image-2.6.35-23-generic, and scrolling is working without any other configuration. It doesn't look like 2.6.35 is available in karmic-backports or updates. So you would either have to upgrade to 10.04 or 10.10 or build your own kernel.
there is a driver for this mouse but it's very recent and you have to compile it yourself: http://github.com/entrope/linux-magicmouse