Actually with fm's replacement library it is quite simple:
(Assuming your constructor is named "lcd")
lcd.backlight(); // turn on backlight.
lcd.noBacklight(); // turn off backlight
It is unfortunate that currently it isn't documented on the wiki and the examples on the wiki
and most of the supplied examples are not using it either.
There are also on() and off() which not only turn on/off the backlight but also turn on/off the display as well.
They essentially call display()/noDisplay() before calling backlight()/noBacklight().
Just keep in mind that if you use backlight() and noBacklight() or on()/off()
then the sketch will no longer compile or work
with original LiquidCrystal library that is supplied by the Arduino team, which probably isn't an issue for
you since they don't supply i2c support anyway.