Okay, so I was 90% of the way through posting for help and fixed my own darn problem, so I'm posting both problem and solution in case someone else has the same issue.
I've been very careful with my new #! install, but seem to have messed something up accidentally.
I removed some applications that I didn't use and edited the Openbox menu using the GUI editor.
Everything seemed fine for a while, but now I can't right-click and see the Openbox menu at all. It's almost like the machine's frozen. Luckily, I have the Application Launch Bar on the panel so I can launch Terminator and Firefox. In Terminator, I can run openbox --debug and I'll get my menu back.
After some searching, it seems like a likely cause is corruption, or malformed syntax in either the menu.xml or rc.xml files.
I checked these and they seemed okay. Didn't validate 100%, but the errors were nothing serious.
The problem turned out to be in the autostart.sh file. I'm a fan of Dropbox and followed pbsMind's minimal Dropbox install instructions in this thread.
The last step is to "Add this line ~/.dropbox-dist/dropboxd & to your ~/.config/openbox/autostart.sh file"
I mistakenly forgot the & symbol.
Put the darn & symbol in and reboot.
Indeed, the dreaded ampersand. See Nik_Doof's idea: http://crunchbanglinux.org/forums/topic … tostartsh/ -- if implemented correctly, it may eliminate this as a problem. It would be good to see some discussion/thoughts on the subject. Probably best to post to Nik_Doof's thread.