Yes and no. I believe apple will allow you to de-authorize all machines on your account once or twice a year. I just looked around Itunes, and for the life of me, I cant find it.
Just checked the Apple Support Website.
How do I deauthorize all of my computers?
If you have authorized five computers, a button labeled "Deauthorize All" will appear in your Account Information screen. This button will deauthorize all computers associated with your account. You can then reauthorize up to 5 computers. Note: You can only use this feature once a year.
Edit.. wow, all those posts arrived as I was typing mine!!!