If the font is absolutely necessary, you will have to go with the text as image solution. As of December, only iOS Mail, Mail.app, Lotus Notes 8, default Mail on Android, Outlook 2000, and Thunderbird support the use of an external custom font.
See: http://www.campaignmonitor.com/blog/post/3897/using-web-fonts-in-email
Edit 2/10/2014: Since this is one of my most popular answers, here is the updated link on best practice of web fonts in emails:
http://www.campaignmonitor.com/resources/will-it-work/webfonts/