I've just had a look at which AppleScript commands iPhoto '05 does 
supports. As it turns out, it does not support the asynchronous 
launch of a slide show like iPhoto '06 (so that's new in iPhoto '05), 
which basically means that - once a slideshow is started - you loose 
any control over it via AppleScript (any AppleScript will just block 
until the end of the slideshow). As AppleScript is needed to 
determine whether a slide show is running or not and then react 
accordingly, iPhoto '05 as such is not compatible with the iPhoto 
Behaviour. It's '06 only at this point.

You can work around this quite easily, though:

1) Create a custom action with this AppleScript snippet:

tell application "iPhoto" to start slideshow

Choose it for the button you want to start the presentation with.

2) Map the left and right buttons to the left and right cursor key 
actions of the Virtual Keyboard

3) Map the esc key action of the Virtual Keyboard to the button you 
want to quit the slideshow, and the space key action to the button 
you want to pause the slideshow with.

Best regards, 
Felix