Roaming Symbol ???

What "roaming symbol"? (I ask because AFAIK there is not one on the iPhone, although there is a pop-up asking if you want to activate data roaming if you your network is unavailable and another carrier's network is available.)

Correct, what symbol. I have never seen one.

There are a few symbols for various things.

No box (no data network)
Box with dot (use to be just box with no dot in old firmware), means ... forget the term, but slower than Edge.
Box with E (Edge)
I don't own 3G but think it is a 3G or a G or something like that in the box for 3G

Here are the connection symbols for 3G, EDGE, GPRS, and WiFi. Other status bar symbols can be found on p17-18 of the iPhone User Guide.