It usually only shows up next to the wifi symbol or the "3G" symbol. But not for any of the other network connection types. Even with BIS.
The 2G data connection types ("EDGE" for GSM service and "1XEV" for CDMA service), don't display the BIS symbol. Instead, simply being all caps indicates BIS connection. If those are small letters (like "edge", "1x" or even "3g") , then you are on the carrier's mobile network, but not connected to the BIS servers at all.
A somewhat related question that I have... Why does that BIS logo symbol use only 4 of the 7 dots that make up the full BB logo?
Posted from my CrackBerry at wapforums.crackberry.com
Lowercase letters mean that you're not connected to any data network, it's only when their capitalized that you're connected. Doesn't matter whether it's 1x, edge, or 3g, lowercase means no connection.
The BB icon indicates that you are connected to the BIS. It is possible for the BB to be connected to the carrier network (capital letters) but not connected to the BIS. Making sure that the data plan is the BIS plan, and that the service books have been sent to the device should resolve any issue.
As to the icon only showing when connected to WiFi and not when connected to 3G, I'm not really sure... Seems strange to me. The only thing I can think of is that with OS6 they may have moved the location of the BB symbol from next to the 3G symbol to below it. I can't remember how it is on the Torch, but I believe it is that way.