iTunes Music Store and Deauthorize All
Apple Knowledge Base (via Mac OS X Hints):
If you find you have reached 5 authorizations due to system upgrades, you can reset your authorization count by clicking Deauthorize All in the Account Information screen. Note: You may only use this feature once per year. The Deauthorize All button will not appear if you have less than 5 authorized computers or if you have used this option within the last 12 months.
