I have a Samsung Galaxy S II and my phone provider is Sprint... It was a royal pain that I could not reply to all when I received a text message that was sent out to a group of people... I have found that if you download Handcent you can send/receive and reply to all in group conversation threads. Here's how:

1. Download Handcent SMS
2. Go to the Settings Menu > Application Settings > select Default Messaging Application (Select All SMS & MMS)
3. After completing step two, select Thread Group Conversations (Note: You must select Handcent as your default message app. before attempting to thread conversations)

Hopefully this helps other Sprint/Android customers! (It's a minor thing but it can be a real pain )