I've just installing and configuring Synergy (a program that enables you to share one keyboard and one mouse for use in multiple machine) between my Ubuntu Box and Powerbook G4. Actually, it is so simple, all you need to do is have a synergy installed on both machines.

1. Ubuntu:

$ sudo apt-get install synergy

2. Powerbook:
On Powerbook, there is a client called QuickSynergy, but I have a random disconnect problem using it, so I'd rather suggest you to installing Macports, and do this:

$ sudo port install synergy

Choose either one as your server and the other as a client.

*SERVER
In your server, you create synergy.conf, containing this:

section: screens
            ubuntubox:
            powerbook:
end
section: links
            ubuntubox:
                        right = powerbook
            powerbook:
                        left = ubuntubox
end

Then, still in your server:

$ synergys -f --config synergy.conf

And, if it run properly, you can see it in messages that show up, do this:

$ mv synergy.conf ~/.synergy.conf

Then, next time you want to execute your synergy server, just do this:

$ synergys

*CLIENT
And from your client, do this:

$ synergyc -f [your-server-ip-address]

I'm sure you already can have your mouse running around between your two machines :) Enjoy!