does anybody have experience with the D40 as an off-camera flash a la strobist?
Yes.
I use two D40s on stands fired by Pocket Wizards and powered by Quantum Turbo Cs for shooting wedding formals.
Of course used with Pocket Wizards, there is no TTL ability, or ability to adjust the power ratio (it's always "full power"). But I couldn't care less about that as I usually need all the power they have anyway when firing through translucent umbrellas or off reflective umbrellas.
To control lighting ratios, I simply move the parabolic reflector from wide to normal, and/or literally set the stands at different distances or move the umbrella shaft in and out. I'm usually placing the stands differently for effect anyway.
There is a 16' hard wire TTL extension cord that plugs into the existing D40 TTL cord ... and you'll also need a 16' sync cord. Then you can shoot full TTL with a D40 off-camera. This works quite well for studio type stuff with a bounce surface for fill. I vaguely recall that they also made a "splitter" to hard wire two D40s for off-camera TTL.
The only reason I still use these is because they are bare bulb, and I had them left over from my V days. I use them this way interchangeably with any camera ... Leica M, Sony A900 or the H camera. Even if I could set up TTL, I probably wouldn't because I like to specifically control the lighting myself. TTL off-camera is over-rated IMHO ... you just end up over-rideing the settings anyway.
If you are in the US, and want the 16' TTL cord and 16' sync cord that I have, then PM me.
In fact, if anyone wants the whole set-up: two mint D40s, extra battery modules, Pocket wizard cords, Quantum cords and the TTL extension cord, let me know. I'm moving to just manually using my Sony HV58s with Pocket Wizards to cut down on my wedding kit size.
-Marc
Here's one ... D40 fired into reflective umbrella set camera left as main ... second D40 and umbrella set 2X farther away and feathered to just provide subject fill, and light the background.