I found the Developer Mode Enabeler app in the webOS Doctor archive, set Visible to true in the appinfo.json file, tested on the emulator and then installed it on my Pre.

I didn't believe it would work -- because we would have heard of it ages ago -- and it doesn't. Whether my Pre is in Dev Mode or not, my app (Dev a la Mode) says that it isn't. I switch it to on and it tells me I have to reset the device and I click it and ... nothing. It goes back to the main scene like nothing ever happened.

I've turned Dev Mode off and back on and installed another app, so my experiement didn't cost me.

Without my Pre being rooted yet (I'll get to that eventually) it was worth a shot. Still, why didn't it work? Any guesses?