Sorry for not replying sooner, but I just came across your question.
MailShot Pro will allow you to do just this- it creates special contacts you can use from Mail, and tens of thousands of compatible apps like Photos, Pages etc. When you send an email to the MailShot contact, it gets sent out to the entire group. You can turn address book groups into MailShot groups, and over 250,000 people are now using it for group email.
Compared to other group email apps it has two advantages- you can use it right from the Mail app, and you can forward emails to a group, including all attachments.
You can try it on the App Store at http://jo.my/mailshot
A free version is also available to try, which only addresses smaller groups.
Disclosure: I am the developer of MailShot Pro and may benefit from its sale.