B757 Captain experiences a Left Engine Fuel Filter EICAS message shortly after takeoff and after declaring an emergency; returns for an overweight landing. Overweight landing information could not be found in the new QRH.

Narrative

[We had a] normal engine start; taxi and takeoff. At approximately 500 FT; LEFT ENGINE FUEL FILTER EICAS warning message appeared. Continued takeoff profile; selected VNAV at approximately 1;000 FT. Completed after takeoff check and referred to QRH to find appropriate checklist. First Officer was pilot flying and used autopilot while I completed checklist and consulted with Dispatch and Maintenance. [We] asked for vectors to return to the airport. On leveling at 9;000 FT; EICAS warning message went away; but Status message remained throughout flight. Maintenance concurred with return to field; and neither Maintenance nor Dispatch had any guidance/comment on overweight landing. [We] could not locate any info in [new] QRH regarding overweight landing. Gave the QRH to Captain who was on jumpseat and he couldn't find anything either. [We] declared an emergency with Approach; reference possibility of engine failure due fuel starvation. Briefed flight attendants for normal landing and made PA announcement for passengers. First Officer flew ILS and landed normally; Vref 140 KIAS and landing weight 220 K. [We] cleared runway with minimal braking. [It] would have been nice to locate something regarding overweight landing but unable to do so in time available.