on my Iphone 4 3g what does the little grey cloud with the 3 dots mean?

if you mean the little cloud thing when you are using IMessage that mean the person you are messaging with is responding back to you in other words you know when they are answering you back lol🙂

I'm guessing you might be referring to the three dots that flash for a while, showing that the responder is typing an iMessage response.

Curious if you ever see them, but then don't get a text. Wondering what this means

It could mean that the other party started typing you a message and then changed their mind.

It means that the other party in the conversation is using the messaging application. They may be reading or typing but there's no guarantee that they are reading or typing in a conversation with you.