I wanted to cancel my account as well, for reasons of my own not open to discussion, so I emailed AOL support asking them about it. The reply I got is as follows:

Thanks for writing to America Online and for making us aware of your concern. I
am Mervin and I will be working with you regarding your email.

1. Highlight the screen name in the AIM Sign On window Screen
Name box.
2. Press the DELETE key on your keyboard.

This will remove the screen name from the software but not from the Instant
Messenger service. The Instant Messenger registration server keeps the screen
name reserved for 12 months. After 12 months time, the AIM system will release
the screen name you deleted for re-use by America Online and Instant Messenger
members.[/b]

Mervin V.
Customer Care Consultant
The Techmail Department
America Online Inc.
So there you have it. AOL claims that after a year it is recycleable.