What does a "o" symbol instead of "3G" or "E" mean?
You are connected to GPRS/1xRTT since other network connection types are not available.
iOS 5.1 status icons
On GSM (All but Verizon):

3G = UTMS/HSDPA
E = EDGE
o = GPRS

On CDMA (Verizon):

3G = EV-DO
E = no equivalent - not possible
o = 1xRTT

(iPhone User Manual on Page 12)