Within iTunes you can only have 5 machines authorized to the same iTunes account, meaning you can have up to five computers that can play the songs that you’ve purchased. Well some people will be upgrading to leopard on their older machines and prefer to do a clean install. While for a windows user this could potentially be disasterous if they did not de-authorize that computer before wiping, it is of no consequence for the Mac user.
Apple authorizes (as far as I know) based on your Mac’s serial number. This is great the the mac user. Once you have installed the new OS and restored your data from your backups (You do backup, right?) All you have to do is Authorize your account on that machine and You should not see your Authorized computers number go up at all. That’s it…something to keep in mind in case you need to wipe your Mac and start over.