there is an app in cydia called GroupSMS that willdo what you want, i havnt used it because when i sent a group sms its a different "group" of contacts everytime so i just add them manually, but the description looks like its a good app
I was also looking for a group SMS app that allows personilization, the only one I found is called Dear X. It is in the app store. It does the trick but the layout isn't too nice in my opinion.
I'm going to check out Group SMS now to see if it does it