I think the smallest laptop you can get with a full size keyboard is a 14" widescreen. With that in mind, stay away from notebooks with dedicated graphics, since even the low-end cards suck up a lot more juice than the integrated graphics chips.
Notebooks to look into (all 14" widescreens):
HP nc6400 (~4-5lbs), battery life with integrated graphics (from a review) was about 3-4 hours
Asus A8 series with the integrated graphics (~5.5lbs), battery life was 2-3.5 hours
Fujitsu S7110 (~4.5lbs), battery life with integrated graphics (from Fujitsu) was 5 hours with the main battery, 9 hours if you add in the modular battery
Various Thinkpads would also fit your needs (either from the Z or T series).
Dell Latitude D620 with an extended battery is another option.