Apple's iOS4 has a convenient new feature tucked away in Settings > Messages called Group Messaging (pictured).
I was trying to figure out what it did, since I’ve always been able to send texts to a group. When I turned Group Messaging ON, it switched the title bar in the text message screen from "New Message" to "Group MMS" (pictured below).
But that was all I noticed -- there was no change in the outbound behavior of my group texts. So I called AT&T and Apple and here’s what I learned:
AT&T confirmed that sending a text to 100 people (via Group Messaging) only counts as one message sent. If I did the same thing using SMS I'd use 100 messages against my balance. It's basically a dirt cheap way to text. Here's a video of how it works.
AT&T noted that none of their other phones are currently capable with Group Messaging. The video highlights the fact the Droid Incredible 2.1 doesn’t do this right now. This looks to be nice benefit for frequent texters looking for less expensive group texting and true group chat built into the OS.