I decided I wanted to reflash my N800 using my MacThing running OS/X 10.7 Lion.

I downloaded the latest Flasher-3.5 dmg, which is compatible with 10.5 (Leopard) and 10.6 (Snow Leopard).

It installed without fault. But I can't find the executable anywhere on the disk.

Where should it have installed it?

Yeah, you need to copy a few files to get it working on Lion. Don't worry, it's quite simple - here you go (in two steps):

1) osx lion puts the usblib files in tmp folder. copy them to usr/lib:
$sudo cp -Rv /private/tmp/pc-connectivity/10.6/* /usr/lib/

2) likewise, copy flasher-3.5 to /usr/bin:
$sudo cp -v /private/tmp/pc-connectivity-bin/10.6/flasher-3.5 /usr/bin/

Worked for me.
Cheers.