Using Nextel 7510 with 4.0. Worked great.
What I did was move the icon to the very last position in the ribbon. That way I can leave it off to save battery, but if I find myself in a dark place and need to use it, I can hit escape a bunch of times to get to the ribbon page (worst case), and then scroll the track wheel down a bunch of times until I'm sure I'm at the end and then press the track wheel to activate the backlight.
I've tried testing it in different scenarios and it's working great for me (at least until there's a better solution).
Thanks. Great app.