I have 4 external keyboards for my notebooks. Apple keyboard with wire, Apple Bluetooth keyboard, Kensington Mac slim keyboard with wire, and Gyration wireless keyboard/mouse unit.
Apple BT keyboard works excellent with my 12" PB as I don't want to plug into it each time I am home as I want it to be easily mobile without doing plug-in & plug-out.
I think wired keyboard is good for more use as you don't need battery and no lag on typing. Also, no battery means it is lighter than the BT or RF keyboard and you can easily move it around the desk with one hand (Apple BT is heavier and you need to grip it well before you move it around the desk).
I just purchased Gyration slim keyboard/mouse unit for HP notebook and this thing has a nice mouse which you can use while it is on the air (you can wave it in the air to use; of course you can use it like a regular mouse on the desk). I hope Mac-compatabile version of Gyration becomes available.