With .Mac and a Firewire cable, it's quite easy (I do this with my PB and the mini).
For instance, my PB17 is the main Mac. So, I use .Mac sync and sync calendar, Safari bookmarks, Mail settings, yada yada yada.
For the mini, I usually reset it so .Mac sync copies the stuff from the PB directly over to it. However, I use the mini to store my full music collection (about 54GB) I also have it connected to an external hard drive. If I import any photos over to the PB or add any new albums not from the iTMS, I use the Firewire cable and copy stuff over directly to the mini, replacing the iPhoto library and adding the new songs. Quite easy.
Of course, with the latest tweak to Backup 3, I believe you can now safely do a backup from one Mac and restore that backup from another Mac. Haven't tried that yet, but if it works, that'll make it even easier.