When your employment ends with one company, it is important to purge all your personal information and connections from the employer provided machine.
Unsubscribe to as many Mailing lists as you can at work.
Update contact information on company databases, web kiosks, contact lists.
MobileMe: deauth computer, stop all syncs after final clean sync, purge data
Adium: backup all the chat/message logs and then purge them from the machine, delete any accounts that are not work related
Mail: remove personal accounts, purge mailboxes, remove outbound mail relays, scrub rules and signatures
Address Book: backup and purge personal addresses, remove personal search feeds (LDAP, Open Directory, CalDAV, etc)
iTunes: deauthorize work computer, remove stored password and account name, back up and purge any local music/podcasts/videos
Preferences/Networking: “forget” any known personal wireless networks.
Terminal: purge .ssh/config tunnels/proxies and any stored keys
Dump all web browser caches; clear Forms Complete, stored Passwords (not all browsers use keychain), purge cookies (Firefox, Safari, OmniWeb, Chrome, Camino)
Dropbox: remove your dropbox and XMarks and Delicious sync credentials
iCal: remove personal appointments, remove sync
Skype: clear stored accounts.
Remove any KeePassX files.
Update the outgoing message on your company voice mail box.
Personally Licensed Software: Graphic Converter, OmniGraffle, Things, DiskWarrior, OmniWeb, OmniFocus, etc – remove license keys and registration.
Keychain Access: purge all stored passwords and keys
Files: check folders in home directory for personal stuff; clean it out, empty trash; ditto on any network shares