Gmail: Send e-mail with different address as the sender

Adding a custom 'From' address
http://mail.google.com/support/bin/answer.py?hl=en&answer=22370

Note to self:
It only applied if you reply/send e-mail from Gmail web UI.
If you're using mail client (such OE), it completely depends on that client app behaviour.
For example, when reply using OE, it will assume that you want to reply an e-mail using the same address it was sent to.