My new app Group Text! is available (and already in new and noteworthy in many iTunes stores). The App can send group texts to groups, user defined distributions lists or handpicked contacts. You can also create and use templates to save typing. iTunes Link If you need a full featured contact manager instead, take a look at my other app ABContacts.
The app is on sale for the week-end. If you're on AT&T and try the app don't forget to turn OFF the iPhone settings option name "Group Messaging". If it's ON the phone will fail to send to large groups and will use MMS instead of SMS. Turn if OFF to send regular group text to large groups.