A B737-700 Fueling Panel crossfeed valve was left open after fueling which removed control from the cockpit crossfeed valve so after a fuel imbalance developed an emergency was declared and the flight diverted.

Narrative

We leveled off at cruise altitude of FL390; and noticed that we had a large fuel imbalance; 5.88 LBS on the left tank; 7.08 on the right tank. Checked positions of all fuel switches and try to crossfeed. The light for the crossfeed would not illuminate but would test as operative (light only). Checked the breaker panel and found the circuit breaker for the crossfeed valve popped. [We] referred to the QRH. By turning off left tank pumps; the fuel slowly began to balance. After reviewing the weather at our destination (less than 1 SM with snow) we decided to return to the departure airport. After coordinating with ATC and company; we started a turn toward that airport. After the fuel balance got to closer tolerances we turned all main tank pumps back on. Fuel continued to burn from the right tank only. At that time I declared an emergency and requested vectors for landing an en route airport. Continued to the divert airport; notified company and landed without further incidence. We landed with 5.63 on the left tank and 4.80 on the right tank. Maintenance told us that the fuel valve on the fueling panel had been left open causing the fuel cross feed breaker to pop and fuel to be drawn unevenly.