The techy answer: Your iPhone is now on the GPRS data network. The English translation: “Sorry for the really slow data speeds.”
In a perfect world, all smartphones would always have access to fast and reliable 3G networks. Of course, the problems with often-overloaded network are well publicised, and anybody who’s ever tried to use a phone in a dense city knows that slow speeds are far from unusual. So when a 3G network isn’t available, 3G-capable phones regress to the slower EDGE (short for Enhanced Data rates for GSM Evolution) network, and will let you know by displaying a capital “E” at the top of the screen where it usually says “3G”. But what if even EDGE isn’t available? In those all-too-common cases, the phone kicks it down one more notch, and you’re left with GPRS – or General Packet Radio Service. This slower data network – signified on the iPhone by the circle in question – should suffice for sending and receiving simple text e-mails and is certainly better than no signal at all, but don’t expect it to be very useful for surfing the Web, streaming music or running data-intensive mobile applications.