I spent six months last year writing explanations for a standardized test. It was extraordinarily boring. But now I get $1000-$1500 per month in licensing fees.
I think there are lots of passive income opportunities for developers to make passive income in non-technical niches.
For instance, I've only been learning programming for a few months, but I have a Wordpress blog. I'm probably going to buy that plugin. Always wanted to track that, but didn't know how.
I've had adsense sites making a few hundred $ a month since 2005. Its hands off to the point I don't even know what they made until I get the 1099 from Google each year.
However every (or nearly every) business that makes money truly passively is an under performer. They are like boats without captains that have not sunk yet. Those same sites, along with the A list domains some are on should be earning $200k+ a year by now.
In that sense, I would define passive income as doing a terrible job on a stellar business model.
Of course, there are also businesses employees or partners can run. Those absolutely can be done. I have one project like that right now. But, just like a dividend paying stock or bond, you are going to have to put in a serious asset to minimize the chance of the people running the show from fleeing.
And I also want to point out that nobody is talking about "wealth" around here, just about a few more bucks.
Cue stories of the masses who had wealth rained down on them by doing very little work: