B757-200 flight crew failed to note the fuel crossfeed was open until on a visual approach to their destination. Are informed by a low fuel EICAS message; a go-around for evaluation and an eventual uneventful landing.

Narrative

While on base leg to a visual we received a low fuel message. The right main fuel tank had 2.2 remaining. Upon examination we discovered we overlooked the crossfeed switch during preflight and it was open. The entire flight was fed from the right main tank; over 12;000 LBS; leaving a 12;000 LB imbalance. We started feeding out of the left main tank and elected to discontinue the approach and contact Maintenance Control. We were not going to burn down to the limitation limit since that would leave us with less total fuel than we were comfortable with so we wanted to know if there was some maintenance information that would prohibit us from landing with this imbalance and possibly damage the aircraft. Maintenance Control knew of nothing unique from our procedures and since we were going to be landing outside the limitation we elected to return to the airport and land with our current imbalance. On short final we went tank to engine and the landing went smooth and uneventful with just slightly more right aileron than normal. In the blocks we had 1.9 in the right main and bit over 11 in the left. During the flight we did get the fuel CONFIG warning but the center tank quantity fluctuated around 1.0 to 1.2 and the light continually illuminated allowing us to discount its significance. I do have the totalizer in my scan but I overlooked the respective amounts in each main tank. The fix for this is real easy; do a complete and thorough preflight cockpit setup and it's probably a good idea for both pilots to double check the other with something as important as the fuel quantity.