If you own a Blackberry, you know why they call them Crack-berries, because once you start using them you quickly become addicted to the instant email notifications, and the ability to easily type a reply from anywhere you have wireless phone service.
By default, your Blackberry service will include a "Sent via Blackberry" line at the end of each email message you send from your Blackberry device. You can change this to make it more personal, or to include contact information. You can also remove it if you'd rather hide the fact that you're using a Blackberry to send email.
You'll need to log onto a web browser in order to change your Blackberry email signature, which is the case whether you're using the Pearl, the Curve, or the regular Blackberry phone.
For AT&T customers, log on to your account at https://att.blackberry.com/ or create a new account using a login of your choice. Once logged in, you can add multiple email accounts to your Blackberry service by providing your email address, username, and password. I was able to set up both my work email and my personal email.
Once you've created an email account, you'll see a link for editing your account. Click "Edit" and you'll see a field for entering a custom signature line. Make your changes and save them, and the next time you send email from your Crackberry, your message will include the signature line at the end.
You won't see the signature line from your Blackberry mobile device when you're composing your email message, but be assured that it will be added to each outgoing message. You can even send an email to yourself from your Blackberry to test it out and make sure it looks the way you expected it to.
Here are the email account configuration links for a few other Blackberry service providers: