http://mail.google.com/support/bin/answer.py?answer=20616
How do I customize the 'From:' address on outgoing mail?
If you'd like your outgoing Gmail messages to display another one of
your email addresses in the 'From:' field (instead of your Gmail
address), just follow these easy steps:
1. Log in to your Gmail account.
2. Click 'Settings' along the top of any page, and then select the
'Accounts' tab.
3. Click 'Add another email address' in the 'Send mail as:' section.
4. Enter your full name in the 'Name:' field, and the email address
you'd like to send messages from in the 'Email address:' field.
*Keep in mind that each time someone replies to a message you
send using a custom 'From:' address, the reply will be delivered to the
'From:' address rather than your Gmail address. If you'd like replies
to be delivered to another account, you'll need to enter a 'reply-to'
address. Just click 'Specify a different reply-to address' to enter
this information.
5. Click 'Next Step >>,' and then click 'Send Verification' to
complete the process. Gmail will send a verification message to your
other email address to confirm that you'd like to add it to your Gmail
account. You'll need to click the link in that message, or enter the
confirmation code in the 'Accounts' section of your Gmail account, to
complete the process. Once you've verified that you'd like to add the
address to your account, you can start sending messages using your
custom 'From:' address.
Tip: If you have a number of different email addresses, enter all of
them in your Gmail account, and then select a different address to send
each of your messages from, depending on what you're sending.